My rules are:
1- Change your own brakes.
2- Do your own lubes and check fluids regularly.
3- Change your own air filters, wiper blades, and other high mark-up disposables.
4- Wash and wax your own vehicle.
5- NEVER try to rebuild your own transmission. Leave it to somebody that looks at transmissions all day.
6- Oil changes are up to you (because it is cheap and fast to get somebody else to do it and get messy), but if you're going to pay somebody to do it.... watch them the ENTIRE time.
Other than that I pretty much just play it by ear. I'd never let a shop do my brakes though. I've seen way too many foul-ups from dealers especially on brake jobs that I've had to try to repair for people. Mainly they tend to strip out the caliper bolts or snap off lugs, or else overtighten them and end up warping the rotors. They also don't like to make sure they get the pads on the correct side so they may have mismatches (some vehicles take one thick and one thin per wheel, but they may put both thins on one side or both thicks on one side if that makes sense). They want to use an air hammer for everything, and brakes should be done BY HAND with a torque wrench to factory specs at the end.
Once you do your brakes the first time and follow factory guide specs you'll never trust anybody else to do it. Plus it may take you 3 hours on the first wheel, but you'll get exponentially faster from there on out. I can do a complete brake job on a front end in under an hour, and that is totally taking my time, torquing properly, and cleaning the garage after.