We use only flat sheets in student clinic, but I much prefer the fitted bottom sheets I use at home or when practicing on my own. The flat bottoms are extra work, and when there's limited time between sessions, I would rather spend my time doing something other than fidgeting with sheets. Once flat sheets are in place, either tied down with table legs or tied to each other, it's really difficult to get a bolster under, or even move it around once it is in.
I find that the twin bottoms are really loose on my table, but the top sheets that come with a standard massage table set are too small. I've been combining the fitted from the massage set with a twin top sheet and like how that works. I don't have a fleece pad, but the table warmer fits no problem.