Hi there! For me, it's always different for each project. Sometimes I collect fabric to try and "reproduce" a quilt or achieve a look that I really like. Sometimes I find some really fabulous fabric and I go hunting for a quilt design that will really showcase it or use it in an interesting way. Sometimes I'm just kinda "in" to a coupla color schemes...and then sometimes I find a quilt that I try to achieve interest and intesity through value. The only thing that is constant, is I don't generally think "color" or "value" it just kind of happens that way...I get a vibe for what I want to make, and play round until I'm happy!
I know she's a bit over done at times, but I love that Lynette Jenson in her books offers a palette for the "Thimbleberries" look. I think that's so appropriate, because tastes are so different, there is no one recipe for color or value that always works with everyone, and it's hardly fair to say you must do this or you must to that to make a great quilt. I really love to contain my colors and stick with a few colors in a few values, but then there's women like Bonnie Hunt from Quiltville.com who pretty much likes to throw in everything but the kitchen sink (definitely not my method) and she bangs out fabulous quilts!
To make a long post longer, I think there are some methods of choosing color/value that are easier than others, but the second you say you can't do something a certain way, someone will pop up out of no where with an amazing quilt to prove that you can. :)