You and me both, only I taped a piece of paper and used marker under the boot area to tell them the bevels.^^ I put a big piece of white duct tape on each ski with "DO NOT DETUNE ANY PART OF THIS SKI" written in bold magic marker.
My intuition would be to do it more often than you would think if you have high performance expectations.
That said, I have no idea how many base grinds you can get out of a ski in it’s lifetime. If I had a good estimate, I’d think about how long the ski is going to live for and just divide. For me, likely once a year except if something goes wrong, but maybe more often.