This is why people like to trade items and runes to get perfect gems.
Note that the charm's mods are based on it's ilvl, so make sure you roll charms that are found past A3 NM to insure the chance of having the + skills mod appearing.
Baal charms, which are charms found from hell baal are worth Um (I think) without any desireable mods, just because they could potentially have any mod possible on them, so make sure you keep any grand charms baal drops.
Ideally roll baal charms, but otherwise I'd stick to ones found in a lvl 85 area, such as Hell Pit or Ancient Tunnels.