Is it rude to borrow an unused power outlet without asking?
Sometimes I'm at the doctor's office, at the library, or even at the grocery store and see an unused power outlet. My phone is dying. I feel weird plugging in, but I feel even weirder asking for permission.
Will you drink a can of Coke™ lying around a stranger's house without asking? No? Then, ask for permission as a matter of etiquette unless there are signs specifically saying it is ok to use them.
Yep, super easy to ask. "Hey, is it cool if I charge my phone here while I wait?" I've never had anybody tell me "no". Assuming you're not also asking for a cable or something, I don't think most people mind at all.