My level 67 team got well over half a level each by doing all the buckets they could reach.

My level 80 team made almost 2,000 gold from 5-boxing all the candy buckets.

For old world, it's pretty easy, at least as horde. Simply visit every capitol city, every noob city (mulgore, razor hill, the blood elf one), and then take every flight point that has an inn at the destination. That's about it. Outland is about the same. Every possible bucket is near a flight point, except the scryer/aldor inns in Shattrath are rather far away from the Shatt FP.