Both, I mix some compost with the castings and layer the bottom of the containers, I also top dress once the heavy flowering sets.
Here's an old story. Way,way back in my early "teen yrs" I worked at a nursery called "Home and Gardens" which is long gone, that's now part of Chicago's Midway Airport parking. Part of my job was to shovel "black dirt" into bushels for the old folks. I was probably wearing a Led Zepplin concert t-shirt at the time, while the "Hippie Dudes" were wearing their "Grateful Dead" t-shirts..........hmmm. They'd run us out of the worm castings ALL the time. The bags were alot smaller back then and the bags actually had a coarser mix of it. By the time I started buying castings myself my manager got suspicious.....I don't know why? I was only growing some Red ..tomatoes. So I quite and cooked in a pizza resturant........
Buying it....pffeh. $60 for a farm and you only need $28 for 500 worms. The instructions say 1000 minimum - I guess that makes better sales, but not sence. They breed like......worms. My farm fills 2 x 1 litre bottles a week. I can't even use that much so I give it to freinds etc.