It depends. If you are happy the way it is then I'd just leave it alone. If you want to use themes to make your phone look/sound different than everyone else's then go for it. There are also lots of other very useful apps available to you when you jailbreak that you cannot get from the app store.
I was weary about jailbreaking my 3g and then I did it. I have to say that after jailbreaking, my battery life went down the hill. I tried restoring it, but unfortunately, it did not restore the battery life I had before the jailbreak. I eventually took the phone back to Apple and had it replaced under warranty.
I say, with 3.0, there really is no reason to jailbreak. I did it mainly to run AIM in the background, but now with push, I don't feel the need.