Personally, I don't put large rocks in my tank for fear of it somehow breaking/cracking the glass (125g). But I also don't have egg crate/etc lining the bottom to disperse weight evenly, only gravel.
My former neighbor had a 75g tank he set up for african cichlids, and he bought some HUGE rocks for that tank. He lined the bottom with something (don't think it was egg crate, but something similar...to disperse the weight evenly).
1 rock was so big it took us 3 people to lower it into the tank, I'm guessing it was 100+ lbs. and before you start mocking us for needing 3 people to place a 100 lb rock, keep in mind its awkward to hold on to, and even more difficult to get any leverage on it while standing above the tank bent at the waist, and trying to avoid getting a finger crushed
His tank didn't leak for the year or two he had it set up.
I personally wouldn't use that large of a rock(s) in my tank though...just too much of a pain in the ___ to use it, move it, or remove it when the time comes.