I've always asked for their defense score. I have a pretty good memory (at least short-term) for things like that, so I usually only have to ask each player once or twice in an encounter.
Mind you, I haven't run 4e for my group yet. When we last played (in April) we were running 3.5, so there was only AC for me to remember. I imagine I'll write down the PCs defenses for 4e, possibly asking them once per session or so if the numbers I have are right (I probably won't remember to update them every time something changes).