There are some great past topics on this here:
Your views on T-shirt Branding
and here:
Name of company on T's
Personally I think it depends on your purpose and target market (and to some extent, your company name
).
Some customers don't mind at all, some will be opposed. If you are really working at establishing a "brand", then the benefits of branding a small logo on the upper back might outweigh the sales that might be lost.
A *big* company logo on the back might be much more off-putting than a smaller logo on a sleeve or back "tab" neckline location.