Its obviously your choice, and greatly helps people who can't pull off any sweet mods if you become a modder for hire. But the more time you work on other people's psps, the less time you have to enjoy working on your own mods, modding your family, or just relaxing. Unless your are totally strapped for cash, I wouldn't trade anything for those few hours you do have. One day you might kick yourself in the ass when one of your kids asked for you to take them to a baseball game or something else and you have to respond "Sorry, I am loaded down with stuff I have to do". Since it wouldn't be right to take smoeone's money and not to the job.
Obviously this is coming from someone who had a busy dad =P. But to each his own.