I'm pretty sure out of my 81 games the number of perfect town sweeps is in the single digits. Not that I would do anything crazy like actually look... but there are thousands of games to gather this data from now. More than enough of a sample size for an accurate percentage.
Also I don't think I ever addressed the question of the OP regarding what should be considered a sweep, but I think a town sweep is a game without mislynches. Expecting no nightkills as well is kind of an unrealistic expectation and I would be surprised if that happens more than once in a blue moon.
