I love buying precuts because of the reason you just mentioned coordinated colors. As for how many to buy like you said one charm will not yield enough for me so never less than 2 or three in case I made a mistake. You can get ideas everywhere for making quilts using them and all free especially Moda bake shop and MSQC tutorials. For me just like all my stash I just buy them whenever there is a good deal without anything in mind but I try to find yardage that coordinates with it at the same time for sashing or border.
Online and yahoo groups are excellent sources for good pricing. Fatquartershop always have some on sale for 50% like these now:
http://www.fatquartershop.com/Charm-...2&Store_id=499
Hope this helps