i never do it. I always reinstall from scratch on any computer I put into service. In the case of my Bionic it is 81 apps from the market, 2 apps that I wrote, and 1 app from someplace else.
This is a great way to assure that you are not "carrying along" a problem form the old machine to the new one.
I think you will find that the Market kept track of everything you had downloaded previously and it is easy to get them again. (I can't be sure because I still have the Droid X that I replaced with the Bionic and I can do everything with the Droid X still except use it for wireless communication.)
... Thom