I've definitely heard of it, and not in a good way.  A guide friend of mine arrived at a BW campsite, saw it unoccupied and went about setting up camp.  An hour later, they found a rolled up tarp at an alternate landing area further away from the fire grate.  Figuring someone left it, they just put it with their gear and go on setting up tents, tarps, bear ropes, etc.  Two hours later a large crew comes ashore, and is quite angry as they believed they "reserved" the campsite by leaving gear there.  Threats were made, so my friend radioed Base and explained the situation.  After a brief rule check, and a call to the Forest Service, it was quite clear that there is no reserving campsites in the Boundary Waters.   Set up camp before you go fishing, exploring, swimming away from camp.  You're better prepared and the site definitely looks occupied with a tent or tarp up.

I've paddled up to crews who looked like they were just eating lunch and asked if they were camping there for the night, and I wouldn't have a problem with others asking me the same.