At my hospital, pretty much the same as all of you. Masks when sterile fields are open, and when the case is going on. After dressings are on, masks can come down. However, I often leave mine up until after extubation, and the pt is on the cart ready for transport. Too many fluids flying at times, you know!!!!

For "clean" cases, such as ESWL, cystos, tonsillectomies, and D&C's, masks are, for the most, optional. Most of use wear them anyway, especially if you are scrubbed.
One rational I heard one time, is that droplets from you could contaminate instruments, like a uterine curette for example, which, does come in contact with open surfaces of the pt.
You know, one of these days, our kids (or kids kids) will look back on us and laugh about how we wore "masks" against our face during surgery!!
Mike