As you can see from some photos of old ads, the printing
on the flip side shows through to some extent.   I've been
framing ads for over 10 years.   I've found that putting
a dark sheet of paper behind will help to hide the unwanted
print from view, if you're really fussy about it.  
I also like to use a mat to keep the ad surface from touching
the glass of the frame.   I tried a hand-held mat cutter
way back when.  Forget about it.  I bought a Logan
Compact Matcutter from the arts & crafts shop, and
it makes it a lot easier to do a professional job.  --Dave