well usually sbf'ing would be to get yourself to a certain version of android. So that u can flash a certain rom. IE: The .340 sbf needs to be flashed prior to install cm7 or miui. because they need the froyo kernel. Flashing from rom to rom doesnt effect the phones performance at all...unless you dont wipe data between the 2 roms. that would most likely cause problems. But in your situation if your running a rom now and your phone is fine. there isnt really any reason to sbf. Iv been switching roms and whatnot lately too. and i havent sbf'ed since i switched from liberty(.596) to cyanogen(.340) To do that i had to sbf to .340 and then install cm7. Since then iv swapped between miui,cm7 and omfgb a bunch of times and my phone is still running fine. i basically havent sbfed in like 2-3 months now