i pick my horses hoofs out everyday (of course,I'm at my horse everyday)
since he is outside right now 24/7 on good pasture. he really just cakes in some dried mud and some rocks in his hoof (which is good ,since it is clean dirt and it helps with filling the hoof and creating frog support which is natural and healthy. so it is probably less likely to develop thrush and other nasties than standing in a stall.
i always try to pick them out before i ride (since that is when you can see if you sprung a heel/bent a shoe, pick out the rocks etc .) and since you will be actually asking them to work and carry a rider, instead of putting them back out in the pasture/stall. i think it is more important.
if i can, i pick after too. then let him back outside to "mosey" and graze. and to let him "repack his hoof for frog support"
if a horse is stalled though. i think you should pick EVERYDAY if you can. or have someone do it for you.
*Member of the Quality Free-Choice Hay/Pasture Feeders Society* *In Favor of Turning horses out as long as Possible*