It really depends on what you're trying to achieve. If you're a good strong climber and not much of a sprinter and are getting top 5's in flat crits, then sure, move up, especially if you have your sights set on some tough hilly road races. Or if you have a lot of teammates who are in a higher catagory and you think you can help them out, then move up.
However, if you're not winning and most of your teammates are in your current catagory, then maybe it's better to stick around a while.
I know of a guy around here who as far as I can tell from his results never won a race as a Cat 4, but had several top 5's who just won his first race as a Cat 3.
I plan to move up to 3 as soon as I can. After that it doesn't matter too much since I'll mostly be racing in 35+ 1/2/3.