For my Commander decks, I personally only proxy cards that I own - and after losing my first Gilded Drake, I will never put a real copy on the table again. I will say I am about to break my stance and proxy one card I don't own, but that's because there is only one legal Grixis commander for my Tiny Leaders cube, and I'm not going to spend $70 on a card I don't even like; the instant they print another cmc=3 Grixis commander, I'm swapping it.
Most of the people I play with also only proxy cards they own.
But occasionally we get someone who plays several, and we don't make a big deal of it, so long as they are legible. We did have someone once who scribbled card names on other cards for 50+% of the deck, and we told them after we wouldn't play that again.
It really just depends on why people are proxying. If they want to play without paying anything, that's a problem; you can build budget decks that are still fun. If it's to run a card in more than one deck without wasting time swapping sleeves, I completely understand. If it's to abuse the proxies and run all the most powerful cards - well, those people don't sound all that fun to play with anyway.