You should give perception to a character that can actually benefit from it, like a rogue or archer type of character, sadly perception is rather useless for all caster and melee classes. The character with perception will spot the trap/object if he or she comes withing a certain radius of it, so if he or she is last in the marching order it might be to late to spot it.

With regards to Fragonard's question:

http://www.larian.com/forums/ubbthreads.php?ubb=showflat&Number=510053

With kind regards,

Rashar.