Anyone ever compared speedlight AF assist on vs. off sharpness?

Today I was using my new 7D and practicing with my 17-55mm F2.8 IS lens and making sure it was dialed in and focusing correctly. Anyway, I was having all sorts of issues with front focusing and back focusing at each end of the focal length spectrum. When one was dialed in the other would be way out...you know how it goes. Anyway, I usually ended up using my 580ex II speedlight to provide better light so I could use ISO 100 and judge sharpness better. Anyway, long story short, I found I could dial in nearly perfectly the lens when the speedlight af assist was off, but when it was on then the difficulty with 17mm behaving differently than 55mm came up.

It's interesting because on my 50D I had been using back button af (in servo mode) all the time so this wouldn't have been an issue as AF assist doesn't work during servo. I just thought it was an interesting find and may help others if they hare having trouble dialing in AF microadjustment on their zoom lens and happen to be using a speedlight.

Upon a second test I think this may have to do with focus assist in close range situations where the beam cannot angle down far enough to be seen. Since I can't seem to reproduce my previous results all of a sudden...do you guys here leave AF assist on(w/ speedlight) or turn it off?

I turn it off, always. I usually prefer to manually focus in low light anyway, and the assist beam can be quite annoying to your subjects.

On the other hand, if you want them looking and to know that you're about to take a photo, you can leave it on as a visual aid to your subjects.

I use the Speetlight AF assist and turn off the flash. It seems to help in more difficult lighting situations. I use it when shooting concerts sometimes. Not sure if the sharpness is any better but it seems to help the AF find it's spot.

Trying to micro adjust troublesome zoom lenses is one of the best ways to end up in the nearset asylum anyway.