I went straight out of undergrad and did well, too, but I still wish I had taken some time off. Even beyond the question of whether you'll do well or whether you'll lack maturity, something that most people don't consider is that once you graduate from law school and start being a lawyer, it's really hard to take a significant amount of time off (i.e. two years to travel, work in a completely different field, teach, etc.). If there's something you may want to do, go do it now! It's the best opportunity, and if you're lucky and really love being a laywer, you really won't have a lot of time (for a looong while) to go out and do something different.
Then again, there's certainly something to be said for momentum. I did well my last two years in undergrad and wanted to jump right in to law school. I always wonder whether, if I had taken time off, I would have done as well when I eventually went to school (or whether I would have eventually gone at all ...)