In my campaigns, I treat cleric spells as requests to their deity, and therefore are not guaranteed to work as planned. Sometimes a cleric playing exactly as behavior would dictate for their class/domain will get rewarded with a healing word automatically maxing rolls, other times the heal will be automatically 1's, or worse due to the clerics wavering ways. This does not happen often,  but over the years of playing/DM'ing this way had allowed the priests in my campaign to role play more actively, as well as research choices much more carefully. 

Nice - I like a little thought in the rpg and not sticking to what the books say....