I stop looting when power leveling in old world instances: RFC, SFK, and SM. And when I have a bank alt full of sub 50 enchant mats.

I start looting in BRD, LBRS, Scholo/Strath just for the Enchant Mats. These are needed to overcome the 275-300 Enchant skill ups.

Definitely loot all of Outlands. Also, it's better to have your alts/noobs to train even 1 skill point in their chosen professions. This way, when rare recipe drop, they can loot it even if they aren't maxed in the trade skill yet.