my vote is drill it yourself, and yea, most of the time the sides arent, and the bottom is.. if you can check with the manufacturer of your tank, to be sure... the other option is, do you have a pair of polarized sunglasses, take 'em outside, and look at the back window of a car, see the little dots? now rotate your head left and right, see the dots change? now, look at the tank, do you see the dots? if you do, its tempered, if not, then its probably not tempered...
the dots that you see when looking at glass are from when it was tempered, it was heated, and cooled in a specific sequience, this causes it to break into the little bits, rather than long shards... the process of tempering, causes the dots, and the polarized sunglasses allow you to see them cause of the "picket fence" fenomenon of the polarized lenses, and the fact that the molecular structure of the glass has been altered from that special heating and cooling...
i have a HOB overflow on my tank, and its works fine once its set up right, even with power outages, i dont have any problems(course, i do have my return pumps on a UPS) but still...
if given the option of a drilled tank, vs a non drilled tank, i'll take the drilled one every time... (i can allways plug up the holes if i dont want them) but once its full of water and stuff, you cant drill it, so, now would be the time.
sorry, i dont know MH's so, i'll not comment on thoes.