Coming from Lahore, I thought I should chip in with my experience about the word paindu in particular and cussing culture in general in Punjab.
The term paindu even though a perfectly fine term for somebody who hails from rural area in Punjab, is used as a derogatory term in urban centers to look down upon people coming from rural backgrounds. It is also extended for people who have typical rural social etiquettes and don't conform to the urban lifestyle completely/are not up to pace with the modern social trends.
I would say in general, people don't take too much offence with that term but it should be used cautiously; you don't want to mentally scar somebody who might be quite sensitive to such mocking. Yes we Pakistanis in general are more thick-skinned than most others but still you should be careful.
I have friends who have rural background and some of them have had really struggles competing academically and more importantly socially with the urban people. I really respect this aspect of their make-up as who they are now. This doesn't mean that there is no leg-pulling and juggat-baazi; we are very much adept at dishing typical Punjabi mocking to each other but I personally refrain from taking jibes at friends who have had rural up-bringing (Ok, if the subject has no qualms whatsoever with you saying it, then of course you may use it; two of you can joke about it and laugh together). On the other hand, I don't let go any opportunity to mock those friends who have had more of an elitist exposure growing up (posh lifestyle, private schooling, etc.) as I feel it is only just that people coming from upper classes experience the mocking as well as they are the ones dishing it out mostly, to people from humbler backgrounds..!(All in good spirits of course)
Coming to the point of who it is mostly used for in ethnic terms, then I would say it is definitely reserved for Punjabis only. I have seen people from Karachi use it as a derogatory term for Punjabis with 'questionable' social manners. Just like most Urban, upper class Punjabis, even hearing two Punjabis talking in Punjabi would make them paindoo in their eyes. I have a friend from Karachi who says that whenever he hears us or anyone else speaking Punjabi, he cant help but think that it is a very vulgar language! (Goes without saying that the first time he said that he was grilled extensively about this statement by a group of five others who all happened to be Punjabis!)
Talking about cussing culture, I would say over the last decade or so, cussing and swearing has started to become more and more common in most Pakistani languages. These days, people in general can't complete a sentence, especially if they want to emphasize on the meaning, without adding a host of profanities. I cant say for certain if Punjabis curse more than lets say Karachiites. I have limited sample of people from Karachi here in where I live so probably would have to visit Sindh and especially Karachi to compare. However, Punjabi do accommodate curse words more easily than Urdu.
Urdu definitely has been historically a more polite and cultured language and most of the severe cuss words actually are not Urdu-rooted words. Those cuss words have originally come from different languages and now started to rear their ugly head in colloquial Urdu language. Personally speaking, I am not a fan of cussing even during light-hearted conversations and such language was never used even in the male-only, cousin-conversations in our family.