If you drill correctly, you put almost no pressure on the glass. The drill should have only enough pressure on it to make solid contact. Diamond drills do their work by abrasion, not by shearing like a normal drill. I guess it all depends on technique.

BTW- someone told be that polarized sunglasses will show you the difference between tempered and non-tempered glass. They said you can see the stress patterned in tempered glass with polarized lenses. I have a 125 G tank with non-tempered glass, so I'll check it out.