Personaly I could care less if I ever ground another stump. I would just as soon cut the tree down and leave the stump.
Stump grinders are high maintenace machines. Always changing teeth because of the trash buried below ground. Then comes belts and bearings. If you do the work yourself after hours and put yourself on the clock as if you had to hire someone to do it would cancel any profit you made doing the stump,probaly be in the hole if you tracked all the hours and expense doing stump work.
I have never found single stump removal all that profitable after cutting a tree down. Every tree guy around here has a grinder which means your not going to get more than the going rate even though it may not be enough. Basically having your own grinder gives you a leverage to be able to do the whole job, sub work is unheard of in my neck of the woods unless your broke down and need a helping hand from a competitor.
Doing multiple stumps like Koa mentioned is profitable but that work is not an everyday occurance. These are gravy jobs, if there was more of this type of grinding out there I would quit cutting the trees.
I get my stumps done as soon as the wood is off the site. I don't put it off because then the job is completed and billable. The next day it might be raining and unable to finish and who knows when we will have good ground conditions to finish the work . Same goes for putting stumps off till the slow season like in the winter, just never know how a winter will go as far as weather.