ACTUALLY.... I don't use the "on board" flash. I use two hand-held strobes, the second is held by my helper and has a photo cell trigger. After using the same gear for so long, I know what my aperature is gonna be at different distances.
Since I started using digital (last year), I use the in-camera meter with a gray card. I still take the ol' Gossen with me. Old habits don't easily die. Besides, why not double-check EVERTHING you possibly can? I would rather find any problems that day and not later on.
This is something I'd like to improve on. Right now I just use the incamera meter for my Canon's. I shoot with the 550 flash and have had good results But, I think I'm going to try a hand held meter and compare the results.