I do always ask for permission when photographing people and private homes. However, on an aircraft where the public/private issue is less clear cut, IMO sometimes it is better not to ask too many questions first.

I once did an NPR documentary by going into a supermarket with a recorder in hand (to get the ambient noise that provides the signature background sound of NPR reports). We had a goal to accomplish, and we felt that we didn't need to rile the suspicion of the supermarket when the topic at hand didn't specifically pertain to that supermarket at all. My attitude is similar with obtaining photos for a personal travel report.