politeness - How rude is it to speak to a Japanese stranger informally instead of using the polite form?

Are there any native Japanese here that could tell me how rude it is to speak informally to Japanese people I have never met instead of using the polite form? I'm assuming it's more rude the older the stranger is than me?

This came up recently when I was at a conference and ran into a couple of Japanese guys about my age (maybe in their 30's) and they seemed to speak less English than I was able to speak Japanese, so we mostly spoke in Japanese. It's been a few years since I've studied Japanese and I've forgotten quite a bit, but I used to have Japanese friends in college and became more comfortable using the non-polite form so that's what I seem to remember the best. But I did not want to offend these guys so I found myself pausing and stuttering trying to quickly remember how to conjugate things politely and the conversation seemed pretty rough and even then I would pop out an informal verb here and there. If I had spoken informally I think it would have been much smoother and in my mind that would have been better for all of us.

So until I get better, wouldn't it be better to say something impolitely than to not say anything at all? Would strangers care that much or would they at least be appreciative that you are trying to speak even if it's impolite?

Examples:

「どこからきた？」instead of「どこから来ましたか」

「これ何？」instead of「これはなんですか」