Yup, that's one of the better standard answers that I'm aware of. Geometrically related to that is that I think you can put a square manhole cover on top of a square manhole in a way that it's unbalanced and will tip in, but you can't do that with a round cover.
Another common answer is that it's easier to get a cover on if you don't have to align it to the right orientation. I'm not sure that this is actually true in practice, though. Likewise, I've also heard that having round manhole covers means you can roll them around, which is something that sounds like it was said by a person who has never handled a real street-sized manhole cover.
The other sensible common answer is that manholes are round, which sounds like a joke answer until you consider that manholes are actually pipes into the ground, and round pipes are rather better at not caving in from ground-pressure than square pipes.
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Another common answer is that it's easier to get a cover on if you don't have to align it to the right orientation. I'm not sure that this is actually true in practice, though. Likewise, I've also heard that having round manhole covers means you can roll them around, which is something that sounds like it was said by a person who has never handled a real street-sized manhole cover.
The other sensible common answer is that manholes are round, which sounds like a joke answer until you consider that manholes are actually pipes into the ground, and round pipes are rather better at not caving in from ground-pressure than square pipes.