All the rules for this are about to change with iOS 5, so I can't really give you a definite answer, but here is my best guess.
Unless you are syncing a lot of music, movies, books, or other files that do not come from one of the iStores, and if you have solid internet most of the time, you probably won't need to use iTunes, and probably won't want to.
The one exception is that you are going to have to use iTunes one more time on the iPad to update to iOS 5. That and the free iCloud account should keep just about everything synced between your two devices without any need for a computer or iTunes.
There are bound to be exceptions to this, but without knowing everything about your iPad and stuff I can't guess. I recommend (if you're not jailbroke) updating your iPad when iOS 5 comes out, or a couple weeks later if you want to be safe. Get used to it, and by time the iPhone 4S is available you'll have a better idea of what you can and want to do.