Quote -  I just tried using IK on the arms in an animation I was working on.. in the middle of it.. and it worked fine. But then I turned it off.. and it screwed up the characters hands, from frame 1! Everything was messed up!  

If you set keyframes for hands only when IK is on, turning it off removing keyframes. Before turn off IK on hands check again if all body parts (forearm, arm, collar and even chest, abdomen) have keyframes.

Most people advise turn off IK when you conform clothes add morph etc. but when you pose your character, use IK as you prefer. Anyway try to set IK on or OFF in whole scene. If you change IK in the middle of animation it can cause weird effects.
Last tip with IK - if you are going to animate dance set, add toes to leg IK chains.