The best point to start doing that is when you can kill the spawns with only a few hits (less than 20 or so usually). In order to achieve this, you have to know their elemental weaknesses.
The green ones are weak to fire. The fast blue worms are weak to electric, and the armored ones are weak to acid.
To get some good elemental weapons, try farming the last mission of the Armory DLC, there is a glitch you can do to get unlimited armory time, as far as I know its still possible to do it. If you don't want to exploit the glitch, then you can do the missions that give you a second and third crack at getting something decent in the armory. (or you can go online and try to find a crawmerax farm to get a good weapon). Keep in mind anything crawmerax drops will be very high level anyway.