Looks real good. We have nothing but Bermuda around here as well, and on the thick ones, there's just no way (that I've found) to NOT leave wheel marks, especially with a 21". They usually go away pretty quickly, however. The only yards I see that don't have them (if they were mowed with a 21") are the ones cut really short, and most customers don't want that.
To folks that don't know, trying to mow thick Bermuda without leaving wheel tracks is kinda like vacuuming without leaving the marks on the carpet. No matter how bad you DON'T want it to happen, it just does anyway. Bad thing is, it seems like, the nicer they lawn, the worse they show up.
Anyway, Gravy Train, your work looks top notch to me. I'm sure your customers are very happy.