If you use capitalization IC, you'll get so used to it that you don't have to think about it. I agree that RP is more important, but using capitalization IC is a fairly small thing that gives a good impression about your character and the player behind him. It also increases the comfort of your fellow player. From two players that RP equally well, I like more of the one who bothers to use capitalization and punctuation. While RP is more important, it isn't as trivially about bothering to do the one little thing.