I have my step son (who I am not homeschooling, not "allowed"), choose one book that he wants, and then I pick out a few, and he has to choose one from those. Its a good mix that he gets to read "junk", but that I also feel he is reading literature that will advance him as a person, stuff with substance.
Maybe I'm just mean. If my kids ask me to read a book I really dislike, I feel free to tell them I'd rather not read it because I don't like it, or I've read it too many times lately. If they ask, I'll explain what I don't like about it, but usually they don't ask. When I do criticize books they like, I am careful to be specific-- I find the plot too predictable, or I don't think the characters are very realistic, or whatever, I don't just say that it's junk. On occasion I also offer commentary on a story while I read it, especially if the character is doing something I would hate to see in my home (we own a collection of Caillou storybooks, and that kid throws a LOT of fits, and never gets much parenting about them)-- that can make a book more bearable for me.
I definitely select books for my kids-- I think the process of selecting books can be really overwhelming for kids, and that can lead kids to stick with series they know they like, or books with great covers and pictures. They don't have to read the books I choose, but they often will. Somehow having them in the house makes them easier to choose than when we're at the library.