I just wet my finger and hold it in the wind.
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.
-Pete