The phone comes out so quickly that it can feel like part of hearing. You like the song, so you record it; recording seems like proof that you liked it. Yet the small screen changes your job. You begin managing a frame rather than following a phrase.
Give yourself one complete piece without filming. You may find that the detail you want to remember is difficult to capture anyway: the way a crowd settles, a musician's response to a passing noise, the difference between a note heard nearby and one compressed through a phone speaker.
If you do want a recording, asking is a good courtesy. A performer may have a posted request, a preferred way to credit their work, or a reason they don't want a particular performance shared. Respect the answer. This is an approach to being a considerate listener, not a substitute for checking any applicable local rules.
Keep other people in mind as well. A camera held above your head can obstruct the very audience that makes the scene attractive. A close-up of a stranger, especially a child, may add something to your clip that they never chose to contribute.
Sometimes a photograph of a sign or a written artist name is the better souvenir. It gives you a way to find the musician's own recordings later. They get to decide how their work is presented, and you get to hear it without the bus that crossed your microphone halfway through the best part.
