Alex,
You do what you feel is best, but I learned to fly by washing and waxing airplanes. I traded one hour of flying for each airplane detailed. That was a full detail, inside and out, which I thought was a pretty good trade.
The detailing services that travel to owners and clean their aircraft charge a king's ransom...but many owners wouldn't bother if they had to pay that kind of money. If you do well and get a recommendation, and have decent prices, you may find that you can get a small circle of clients to work for.
Every wash job should include a full wax. Clean the interior, too. You'll want to be familiar with what chemicals you can use, and how to clean an airplane. Also get familiar with what can hurt you, and what you can hurt. By example, those little white deposits that gather behind exhaust stacks and augmenter tubes clean off with a little elbow grease...but they're extremely toxic. That's lead, from the fuel, and it can cause some serious harm to you. You should never clean it without adequate protection.
Conversely, be sure you don't contaminate static ports, pitot tubes, etc. Be careful about thoroughly rinsing the surface, to avoid cleansing agents settling into faying surfaces (overlapping surfaces), which could be a riveted seam, inspection plate, etc.
Some owners may want the engine cleaned, which requires certain precautions and care. Get some instruction from a mechanic before you attempt that.
One aircraft for one hour of flying. Pretty fair trade.