Hmm, well, it'll mess up something called the baffle step region of the speakers FR. But to be honest, whatever, just do it. They'll probably sound to warm/thick, but you can't use audessay or what ever you have to straighten it out. Try to get the speaker flush to the wall to avoid something called diffraction.
It's not the best, but not really worse than tight against the wall either. Sometimes practicality is more important and in this case I'd say do it.
Now if you want a true in wall speaker and aren't afraid to really DIY, let me know.