As far as jeans go, I cut off the seams at the bottom of the legs (or a little bit more if the bottom of the legs are terribly worn out) and the waist line, cut the zipper out, and cut along the rest of the seams...you should get four large pieces of denim when you do this (the front and back of each leg). Definatly don't pick out the seams. You get so little fabric out of them that it isn't worth the time picking them out. Is it really worth getting that extra 5/8" of fabric?
You can seam rip out of the back pockets of jeans and use the fabric underneath if you need smaller pieces for your quilt (I got a 4 1/2" square out of the jeans I cut out...you might get more or less depending on the size). Just remember the fabric inside the pocket will be significantly darker than the rest of the jeans. And you could save the pockets and sew them onto a fabric square to make an interesting block for your quilt.
Oh, and cut around any part of the clothes that are terribly worn out or have holes in them. Obviously, you don't want that going into your quilt so cut around that and save the good stuff.