all needed is here:
you just have to make account first.
anyways,good question,why you should do factory reset? ill tell you.
this type of flashing (at lease this USA 2.2 release) does not delete anything from phone.so all your stuff is set up just like it was set up on eclair.
1st thing,many apps are better optimized for froyo so they require reinstall
2nd thing,many of your setting get screwed cuz froyo uses different settings then eclair
3rd thing,because froyo feautures different RAM handling and app2sd,many of your apps may become unstable (they still think they are on eclair)
So,factory reset after EVERY upgrade is a must,or youll end up getting WORST performance then you had in eclair. reset for full froyo effect :P