Amazing how so few people ever do this on Bnet it seems. I can spend hours combing empty pubby cow games and come out with dozens upon dozens of flawless gems for upgrading and rerolling. No one seems to bother, but I average 1 skiller per ~30 pgems. A lot of the time its not worth the rune value of the pgems, but every so often you get lucky and roll that coveted Cold Skiller with 30+ life and make a killing. Not to mention the steady supply of them makes it fairly easy to outfit any character you have with a few of these with a little notice. At any given point in time I have about half a dozen spare skillers laying around on mules that I'm trying to trade (atm I believe the count is one Def. Pally Skiller--yes, I know it's junk--, 2 Shapeshifting Skillers, 2 Druid Elementals, and a Necro Curses GC). Easy way to add to your rune collection, and since I'm a gambler by nature, its fun too.
I only reroll charms from Hell Cows or WSK, but that's probably outta habit more than anything else. By the way, don't discount the AR/Life GCs either--I've made a pretty penny trading half a dozen of those recently (between 90 AR and 130 and 25 and 40 life tend to be the range people are interested in, or alternatively the 12% FHR/100+ AR GCs are nice too).
Nice SCs are good to reroll as well (ideally from Baal, but I find that WSK and Hell Cows SCs work just fine too) and every so often I not only get myself a 7% mfsc, but something like the 6% mf, 35 ar sc that I rolled last week, or the 3/17/19er.