I've thought about this for a long time and I'm torn. Just to be clear, I don't sell, advertise them, plan on starting a business or anything like that with my frames.
On one hand, I love the clean and simple look of a natural with no marks or additions to it. On the other hand, I do like the look of a well placed and distinguishing mark to let people know it's your frame.
To be honest, I don't even know how to go about putting a proper makers mark on one. I've tried on a couple of dud frames, using a cheap wood burner and can never get it right or looking decent. It always follows the grain, slips out, burns to deep on some spots, is uneven etc. I'd hate to finish a frame and flub up a mark and have to go about covering it up somehow.
Are guys using a wood burning stamp thingy to do them so nicely? I have and use a "Handmade by ...." from Lee Valley that I use on other woodworking projects (too big and not appropriate for frames) and can burn onto wood veneer business cards, so I'm thinking this is what some of the big guns are using.