I don't use my cast iron skillets very often and lately I haven't even been using my cast iron grill very much - maybe now that it's winter and I'll be doing less outdoor grilling...

I'm with Mark on this one. If you get non-greasy build-up, you definitely have to clean it off because it's not going to help with the non-stick properties. It will just char and stick to the food. A wipe down is fine after cooking some things however. I always put a little bit of fresh oil on though.

The cast iron cookware I use the most is an enameled dutch oven. No seasoning requirements with that one. I'd like to have a few more pieces actually, but I do fine with my regular and non-stick pans. They all take reasonably high heat and can all go into the oven when needed.