Can I use the flash white balance preset when using any Canon flashes like the 550ex or 420ex?
You can, but I don’t recommend it. Studio strobes work better with the flash white balance setting on the D30. The Canon flashes work better with the daylight preset which is 5500K. So, when using any of the canon flashes, the daylight, auto, or custom white balances are your best choices. However, it’s a matter of personal taste whether you use daylight or flash white balance with strobes and it depends on the look you want.