How bad is it to corrupt the terrain? I mean, assuming that you could actually get an uneven lot to be placed?
Corrupting the terrain can cause any number effects, from merely minor terrain damage and ridges permanently etched into the landscape, to the neighborhood sinking in part or whole into the sea. It's generally a bad thing, although not unrepairable if it happens, if you can do terrain surgery. So, yeah, definitely don't try it on a Mac.