Actually, even if you are recording for yourself, you are ethically obligated to obtain explicit permission first. For legal protection, you should get permission in writing. And if you're going to do that, with the idea of future posterity, why not just get a signed release?

Note that getting permission for personal use means just that; it doesn't grant you the right disseminate those recordings later, when you change your original intentions. Your "feelings" about the matter, or even the feelings of those who'd like to get their hands on those recordings, are immaterial. There's a simple term for what you're proposing: bootlegging.

Now I won't pretend that I haven't encouraged a little bootlegging in my time, when it benefitted me, or that I myself wouldn't love to hear (or even copy) those recordings of yours. But the "greater good" ethical argument just doesn't hold much water, especially when the whole situation could have been so easily avoided in the first place by a little forethought--and common courtesy.