phd - Is it appropriate to ask for recommendations, which I'm not sure if I'd use eventually?

Is it OK to still ask for references, if I know I may not use some of them in my PhD applications?

I wanted to ask the potential referees if they're willing first - but I might not necessarily use some of them, because - for example a stronger prospective referee agrees to write letters for me later on; or that I decide to apply for fewer programmes than I originally intended to.

More specifically:

1. Is it appropriate to approach all (or more than enough) potential referees, in the first place, in one go?


2. Should I keep them (those whose letters I did not use) updated? And if needed, how does one politely do so?