I often do. I'm not sure if people consider it rude, or if I care if they do. I consider poetic thought to be a staple of my existence. Sometimes if it comes, you don't want to miss the feeling. I think it's best to roll with that feeling before it passes. People should respect that.
I suppose it depends on where you are at the time. Most people don't want you to randomly wax poetic when you are at the checkout counter of the grocery store, especially during the busy Christmas holiday season, but waxing poetic may be something you want to try on a date or during a long journey in the car. Just make sure you bring the right kind of wax.
Personally, I think the answer depends on how quickly you can read your 'audience'. If you can read them quickly, and recognize when they really DON'T want to hear your poetry, and then stop, then it's probably not rude at all. But if you can't tell if they like it or not, and you just keep going, then it could be rude.
It's usually considered rude if it's just out of the blue. If you're making a presentation and it's what's called for, that's different. But in everyday conversation, it's not typically appreciated. I'm sure there are circles in which it's perfectly acceptable, but I assume you're asking about random, average folks.
If you have something deep and relevant to contribute, I don't consider that to be quite the same thing, and there is certainly room for that.