In anything where the character has a moment's notice, such as in a bartering match with some other character, I find it perfectly plausible for the character to, for example, covertly slip on a ring or somesuch that boosts his or her CHA; it's exactly what I would do, if I felt like going to the effort.
However, Spot, Listen, and other spontaneous checks (anytime a DM says to make a check, not "Okay, interact, here's the checks you'll use.") should all be done, as L said, just as the characters are at that moment.
Wee.