I always ask for permission, a couple of times I've actually found someone in airport management who is into the simulator, and knows who I am, which makes everything easier. However sometimes it's so difficult that I normally quit before I put too much time into a project. Logos etc need consideration, so it is worthwhile getting permission to use these in writing. For NZWN, I was given permission to use their logo etc, including being able to use it in my marketing, but I was never able to get permission to use the Lord of the Rings themed branding/displays, so that was never included. Somewhere I have a letter from the legal representatives of the Tolkien estate, making it very clear I couldn't use anything related to LOTR.
Getting access for photos can be tricky -- sometimes I can get a ride-along airside with airport security, but failing this I'll take photos from public areas only -- again, with permission. I was not able to get airside access for my largest project, but the airport used a contract photographer when they needed one, and were happy to have him help me. He had cameras mounted on his vehicle, so could get a good coverage airside. However I didn't have any control over time/lighting etc, so results were variable.
Back in the early days working on regional airports I would try to get a flight from one of the local operators, who normally were on good terms with ATC, and could circle directly overhead with me shooting frantically. A helicopter flight works well, although I prefer fixed wing with the door removed.
Now I'm getting too old for all the hassle. Small airfields are a dream -- for my current project I visited the aero club who gave me a yellow vest and warned me that if I was going to walk across a runway I look first
I suspect that things are a bit easier here in New Zealand, though.