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:


Is it appropriate to approach all (or more than enough) potential referees, in the first place, in one go? 
Should I keep them (those whose letters I did not use) updated? And if needed, how does one politely do so?
You can ask several people whether they'd be willing to write recommendations for you.  Do not ask them to do any actual work until you need it and intend to use it.  Recommendations will generally go directly to the institution to which you are applying, so "keeping them updated" doesn't really apply.  You'll never see the letters that get written.

When you are ready to ask for the actual recommendation, be sure you provide all the information the referee needs. Substitute "research" or "lab work" for "classes" below as appropriate.Here is what I tell students:


Tell me what the deadline is!
Include your student number.
Remind me which of my classes you have taken, and when.
How did you distinguish yourself in those classes?
How would you describe yourself? What are your strengths? What are your weaknesses? I am going to have to answer those questions when I write your reference, so the more details the better, but these have to be things I've observed myself.
What are some of your academic and nonacademic accomplishments that I may not remember?
What makes me particularly qualified to write a letter for you? That is, why should the recipient of the letter value it over a letter from someone else?


Be sure to tell how and where to submit the recommendation: online link, address for paper letter, etc.