I actually got  a book that had all the stamps lined up running across the middle- between the return address and the to address. There were 5 or 6 stamps I think.  I thought it odd, but hey it got to me okay.

Generally, stamps are in the upper right hand corner...but i would think as long as you have the tight amount of postage you could put them anyplace you want to that wouldn't block addresses or the media mail stamp.....................

That's correct Sue but if at all possible they should be on the face of the package to make sure they get noticed.

This thread reminds me of one of the earlier Harry Potter books where the Weasley family sends Harry a letter but doesn't know how much "Muggle" postage to put on it and so they cover the entire envelope, back and front, with stamps.

You are free to put stamps on the back, but the postal employee who checks to make sure there's enough postage is also free to miss the back entirely, since they may quite logically decide nobody would put stamps back there. :->