Right. I personally don't like the idea of a logo on the shirts, but I'm new to this game so I could be way off. I think if I were to build a brand that's carried in stores nationwide (Urban Outfitters, etc.) or were selling a very high volume of shirts then my name is already established.... (i.e. bustedtees, tshirthell, etc) At that point people know the company name and it could be seen as a fashionable to have a 'tshirthub' tshirt, so a logo could be widely accepted, possibly desired.
I put a plain text copyright on my design which serves as a warning "hey - this is my design - don't copy" and also as a calling card in case someone asks where they got it. It's very low profile (usually countours part of the image in small font). I intentionally refrain from putting a logo the back of the neckline or sleeve because I think it can be a turnoff - and it adds cost. Also - I'm catering to a vintage market so people don't expect 'corporate' stuff like logo's etc.
Another option would be private labeling your shirts with the logo right on the tag of the shirt....we'll save that discussion for another string though.