I'm very new to D&D, so forgive me if this a recurring newbie question. I'm wondering if it's common (or uncommon) for players to discuss their cantrip/spell picks when creating and leveling up their characters. Seems like only one character needs to have cantrips/spells such as Detect Magic, Feather Fall, Alarm, etc. I'm playing a warlock with limited spell slots. I plan to take Pact of the Tome (3 cantrips from any class) and Book of Ancient Secrets (rituals from any class). It would be nice to know my fellow players' cantrip/spell picks ... though I assume it's best to have cantrips/spells emerge naturally during game play ... because why/how would a character know his/her allies' magical powers unless they've been adventuring together for a really long time. Are there best practices or official rules regarding this matter??

Not everyone enjoys optimizing their own character, let alone their entire party. It isn't rude or unusual to share what new features you're picking up (though the list of spells that a spellcaster picks up, especially wizards clerics, can often be TMI for the noncasters), but asking that the other players run their options past you so that you can optimize might be a little pushy, especially if it leads to you backseat driving their build and telling them "no, don't get that!" 

In practice, more than one Detect Magic isn't too much, different characters may be in different places at different times, or may for RP reasons not just want one person being trusted to fairly arbitrate what's magic and who gets it. Alarm has some interesting creative applications, especially if two people in the party have it... you can use it over long distances to essentially signal each other, by each "pinging" the Alarm that was set up by the other player! Feather Fall is a (very important) spell of opportunity, no guarantee that the one character that has it will be there (and have a reaction free) when someone needs it cast on them... etc etc.

By all means, share what sort of character you plan on building, and that you're planning on learning a ton of rituals. But if another player also wants to learn rituals, there are plenty of opportunities for more than one character having those tools to be useful (multiple Floating Disks, etc). 

My opinion is unless a group of characters have back stories where they know each other they won't work together to build their characters at the start of a campaign.  But the first time they level up they'll have spent enough time together to talk around the campfire and when they're traveling and nothing else is going on to cooperate on which options they choose.

I've found that I have more fun building my characters around the rest of the party as I level up than I have building my character to be "perfect" by himself.  D&D is a cooperative game and it's more fun to me to make choices that enhance the entire party than it is to make choices that only optimize my character.

This (obviously) varies from group to group. It would be a good idea to ask other people in the group how they feel about this and go from there. This is a good conversation to have before you start playing with any group if you would like to coordinate spell selections. 

Thanks for the replies so far! It's such an interesting question. For example, our group has three somewhat tight-lipped players who don't seem to want to share much about their characters. I guess we'll all learn more about them in time! Then there are two first-time players - one picked a spellcaster, the other did not. I helped build the spellcasting character,  so I know the character's list of cantrips/spells. It's hard not to let those choices influence the choices I make for my own character. Ultimately, I like the idea of everything being very character-driven. But I can see the benefit of working together as a party (once the characters know each other better) to ensure all bases are covered.

I am one of three "Full Casters" in my party and I send them text messages to discuss spell selection two weeks or more before we level.  I ask them if they think my spells work good for the party and I ask them if I put "this spell" on my list that "he" already has, would that be a good idea?  For example, we are approaching level six.  I can get Counter Spell, but our Warlock already has it.  I asked the Warlock and the Cleric if they think it would be foolish to add it to my list as well.

One caution though, I would never tell someone not to take a spell if they wanted it.  I might tell them why I think it is less than optimum and discuss it if invited, but I wouldn't get upset with them if they went ahead and put it in their list anyway.

Yeah, I think it's a party preference thing. By and large, I agree that you don't want TOO much overlap there, especially when you're playing a spont caster whose spell selection is limited. But some tables are weirdly strict about cross-talk in leveling/character creation, and you don't want to step on toes. That said, if you prefer to have those conversations, make your opinion clear as well. No reason you have to play at a table that doesn't respect your wants or needs.

My players normally compare cantrips but allow some overlap, as long as each spellcaster has something unique. 

 I played a "God Squad" oneshot once, with each of us a different Cleric subclass. Clerics in 5E are the worst for trying to communicate spell selection among the group, since each Cleric can pick an almost entirely new spell list of prepared spells every day which very quickly leads to Too Much News. I don't think we even tried, and it worked out just fine.

Ultimately it comes down to what works best for your group's dynamic, the key is to make sure no one's feeling like they're being pressured to make character choices they don't want to. There are very few spells that can't derive at least some benefit from multiple casters having it.