well, depending on where you want to go, I guess maybe....but yeah, I'd say do it again, just to see if you get higher
You know, depending on what you're trying to go into, you probably could be fine with a 19. I mean, that's basically a lowest you could want to get, but most colleges will accept that. I got a 22, and while I would like to take it again since I know now I could have taken my calculator instead of a pocket one, I'm going to stick with my 22.
21 is the average IIRC so you didn't do so bad. If you can pull it up to around there I think you'll be golden =]. Just study hard.
Study for it, it actually helps. First time I got a 21, studied the week before my second try and got a 27.