ceb58's advice is spot on. General contractors make money by coercing and cajoling subs into doing work for which they won't get paid for, so be very specific in defining your scope of work, actually itemize how many receptacles, switches, etc you are going to install.
It's OK to install an additional plug or two gratis in the interest of customer service but after that charge a fair price for extras and never do an extra without getting signed authorization first. GCs will complain about you "nickel and diming" them to death, but if you don't charge for these items, it is *you* who are getting nickel and dimed to death.
Remember, even though the GC thinks it's "his job", it is not, he is merely an agent for the home owner. If he doesn't pay, your collection efforts should be focussed on the home owner, not the GC.
All GC's are dishonest, there are no exceptions, some people like to claim they work for honest GCs but the truth is when money gets tight, all GCs revert to dishonesty to preserve their profits. Never believe anything a GC says and always conduct your business in a professional fashion.
In the last 30 years I've worked for hundreds of GCs and my experience is that they can give you lots of work with narrow profit margins but you must always keep your guard up. The key is diversification, don't let GC work be all that you do.