good question
MadBlue said:
As a DM I wouldn't have the players roll. As Madfox mentioned, it tempts the players to metagame since they'll easily be able to figure out what their opponents need to roll in order to hit them or succeed in their saving throws.
If, for example, a party spellcaster casts a spell that calls for a Fortitude saving throw and the players roll for the opponent's saving throw - and it passes the save with a "5", the players will know that effects that call for a Fortitude saving throw aren't likely to be effective. There's really no way the party should know that information, but it would be asking too much IMHO to expect the players to not metagame, especially when they know they'd just be wasting their spells/poison/etc.
MadBlue
I try to keep this to a minimum, this sort of meta-game stuff/calculation, and will call the players on it if it gets stupid.
However, it also might make sense in-game ("you cast your most powerful magic, and the enemy X easily shrugs off your spell"). Spellcraft check? (to see how easily the bad guy resists it?) Can a PC ever "know" that his "hold person" won't effect that 10th level evil cleric except on a very low roll? Maybe. Might be a good flavor effect.
Of course, it's usually moot; most savvy players either know monsters from the MM's and thus "know" the bad saves already, or can figure it out quickly (good fighter=poor will, fast/sneaky = good reflex, magic use = good will).
I'm not sure how much you are "giving away" by rolling in the open. Plus, good players won't metagame, however, if (for example) a fighter is fighting something and rolls a "2" for IT to hit, and it HITS, I would let the fighter (in game) realize that what he's fighting is WAY skilled and VERY dangerous (i.e. "you dodge, use your shield, parry, etc. the attack that you saw coming and it STILL gets through"). Etc.
In general, I don't think meta-game thinking is much worse this way. So far. I'll keep my eyes peeled.