Q: Set white balance with flash on?

When using a flash (580EX) on the D20, and wanting to use a custom white balance, should I take a picutre of my white card with the flash turned on or off?

I recently took several pictures indoors and using a CWB, but when I uploaded the photos everything had a strong blue cast to them. Previous "tests" showed me this was a good method -- boy was I wrong!

What is your trick? Am I thinking too much, and should use the flash WB when the flash is on?

Thanks!

Pat