This is an interesting point though, and something i pondered over recently when i had to go to docs for something...and before that when someone mentioned in a post (might have been katmandu, not sure) that they didn't go to heavy on something in a scene, because they were due for an exam at the docs the next day.
I do essentially the same thing. Though, I have been somewhat extreme in my life and it has left permanent scars and I have slipped my way through with a list of preset explanations (preplanned lies) as to not arouse too much suspicion. As for current or fresh markings, Master is just really careful right before I have to go in to see the doctor. But, I do occasionally panic that I may have to go to the emergency room (not bdsm related but say the day after a rough scene) or to see the doctor the next day unexpectedly and it would make it easier if I had a doctor that I knew "understood" about the nature of these things. Now, if I know I am going to have an appointment, I make sure I have all my "excuses" in order and my Master does his best not to add any extra suspicious marks. Myself, I would love to have some extreme body modification (not talking piercings or tats here) but I am too scared to do it not because of the extremeness but, at this point, for fear that it would bring up more questions. I just do what I can to keep from arousing suspicion but it sure would be easier if I had someone as a doctor that understood.