In reality, the perforations on stamps are not round, but actually slightly oval.

A sheet of stamps would have all the horizontal perforations applied and then be turned and have the vertical perforations done, one or more rows at a time, perforated by a "comb" or a "perforated wheels".
Reperforated stamps, assuming that's the concern here, is really something that must be
read about. I don't know where the "rough edge" of a perforation would show up vertically. This is an interesting, and scary link on the subject.
http://www.philatelicfoundation.org...d_stamps.htmHope this helps
-IBFS