I am sure there are many different policies regarding this. At SKW there is no problem so long as it does not interfere with your max day/week/month/year time. They do ask that you disclose any other flying your are doing for obvious reason. If you fly the Bro you get an extra 20hrs a month you can fly (120 max) and if you are a line holder you could easily CFI 25-30hrs a month on the side. I have yet to run into anyone who does this but it could be done.
Cheers