The readers have chimed in and overwhelmingly agree that an autograph goes on the front. Rod even goes so far as to write in the autograph space asking the player to sign the front. Several readers pointed out other sets which have autograph spaces on the back. Rod pointed out Mothers Cookies, which being West Coast based I have heard of but never seen, and GCRL pointed out a set called Permagraphics which I've never heard of.
Some more mainstream sets include Classic, pointed out by Night Owl. Here's an example from the awesome tiger-striped 1990 set:
Pricelesspursuit remembers Starting Lineup cards having spaces for autographs on the back. I don't have many, but the ones that do have a facsimile autograph on the back.
Those sets had a cool design, but I think the abbreviation O for outfield looks a bit weird.
There's no doubt about it, an autograph looks best on the front!