In general, updating the BIOS is done to fix bugs, improve compatibility with RAM, etc. There's no general need to do it before going to Win7.
If you decide that you wish to do it, I recommend NOT doing it from within Windows. (It'd probably work, but there is a higher risk of a bad flash if you use a Windows-based utility.) Also, if your motherboard supports a method for recovering from a bad BIOS flash, you'd best prepare that before the update. I've never rendered a system unbootable with a BIOS flash, but it can happen. (I once read of a failed flash by a chap whose dog pulled the power cord in the middle of the flash.)