Using a sheet of stamps as a bookmark is a terrible idea for multiple reasons:
You "run out of stamps" and have to get more--but are puzzled as to where you used them all.
You put the book down for a year or so and when you pick it up again the price of stamps has gone up 4 cents which makes using those stamps a hassle (though my fix for this is to just keep waiting and eventually the stamp is the right amount for postcards).
If you used the "licky" stamps, sometimes there's just enough moisture over the period of time it's been in the book that the stamps become parmanently adhered to the page that you bookmarked.
So don't do it! Terrible idea!