The problem is:
most buyers can add the copyspace themselves on isolated obects, others cant because they are no designers and such, they just want to put some text over the image. Some agencies might see that potential, others dont. I guess they know their clients better than we do.
On the other hand: the buyers that can put the copyspace there themselves might be angry if they buy too much white space around an object. So I suggest you produce a very tight image you upload to most agencies and add a little extra white space for uploading to dreamstime in PS.