Do you allow your players to count or measure their movement before they act?  Do you allow them to change their action once they realize their count is wrong?  Especially ranged area of effect spells.  Should it be easy for a wizard to place that fireball right at that point to get more targets?  I do, just wondering if I need to adapt.  

I think it actually states in the DMG that the players can count and measure whenever they want. If they can't place a fireball where they want, where would it even go?

The combat rules portray a combat, they are not an accurate simulation. For a player measuring distance on a map, you need to remember that they are just looking (or otherwise sensing) a creature or object that they can see. I know whether or not I can throw a tennis ball to hit someone except at maximum range. One time I loose an arrow, it might go 200 feet. Another time, it might go further depending on the angle. Plus I have to arc the trajectory, not too high, not too low. We ignore that kind of thing in D&D.

So yeah, you need to allow them to count, measure, and place area effects.

If you are playing on a battle map of some sort then measuring is inherent in play. If you are playing theater of the mind, then the GM should have an idea of where everything is and you can't really measure anything.

Depending on the situation you must let players change their move. If they go too far then it is illegal for them to go that distance.

I have no problem with a Wizard that wants to place a fireball in an optimum location. My Bad Guy Shaman wants to do that too!

Yeah man,

Don't try to make the players flub/waste their actions because of a 5ft grid that is only used to facilitate combat.  That part of the game is very much so gamified to the point where a spells with a radius end up covering a square on the map.  Let them count, measure, etc, because anything else is just having the PC waste their turn/actions/spell slots because the player miscounted squares.

If they spend way too long on their turn trying counting squares to try to figure out what they are going to do, that is a different story and you have an obligation to move things along.

I'd recommend time limiting it so they don't spend all day on it but yeah, if they want to measure that's fine. They just need to decide by the time it's their turn. 

There was a thread not too long back complaining about a VTT rendering a circle representing the area of affect of darkness and it giving a meta advantage to the player because "realistically" the character would likely not realize the origin point and radius of spell to move out of.  Seems sort of germane to the discussion, though personally I think those are the breaks of playing with a VTT or battlemap.  That said, even in theater of the mind if I'm asked how viable a movement is or range is, I recognize that the proprioception of a character via a player through the conduit of a character sheet is what it is, so I grant them the ability to know measure (though I will be a jerk about darkness and fogs and the like, requiring orientation checks and the like).  

But yeah, if you're playing on VTT or Battlemap you've already put a chess type calculating parameter into the way your game performs the rules.  It'd seem to me bad form to block deliberation of measurements and ranges.

As long as it’s not bogging things down, who cares? Does pre measuring speed things up? Measure all day long except when it’s your turn and keep things moving because gorram combat bogs the game down. If the combat is too complex for TotM, then I use a grid and keep it simple as counting spaces. Like, meh. 🤷‍♂️ Ne?

I'd say allow people to count, measure, do whatever it takes to be accurate. We still have to roll die to determine success of attacks anyway, and you can decide at session zero if there's going to be critical flubs that make a flask of alchemist's fire go awry. If they fail to hit, or if an enemy succeeds their saving throw, I'll use that to flavour them not getting close enough/overshooting or whatever. In Theatre of the Mind (ToTM) however, I view it like a JRPG where, mechanically, enemies are practically in a line but there's more flavour to it.

All of this is to say that if I hold players to these standards I'd only find it fair to be held to them also, and I far prefer tactical considerations than sheer guesswork.

Quote from HexTherapy >>

I'd say allow people to count, measure, do whatever it takes to be accurate. We still have to roll die to determine success of attacks anyway, and you can decide at session zero if there's going to be critical flubs that make a flask of alchemist's fire go awry. If they fail to hit, or if an enemy succeeds their saving throw, I'll use that to flavour them not getting close enough/overshooting or whatever. In Theatre of the Mind (ToTM) however, I view it like a JRPG where, mechanically, enemies are practically in a line but there's more flavour to it.

All of this is to say that if I hold players to these standards I'd only find it fair to be held to them also, and I far prefer tactical considerations than sheer guesswork.

Sort of building on this fair play point at the end, if the DM built the battlespace using squares or measurements to plan out the encounter, the players accordingly should be able to also calculate their response.  Otherwise it's sort of like playing chess against someone who's not allowed to know the rules of how their pieces move.

This is not to discount fog of war, but off VTTs, Fog of War is challenging on physical battemat unless the DM wants to really reduce the possibility of the environment's structure, but that's starting to drift off topic of the thrust of the OP's question.  Basically if combat is played with a looking down from above perspective, I see no wrong and much right with counting squares or using range sticks or circles.

Count... measure... as much as you like & let the dice sort it all out in the end.