I am one of those "allways tweaking" kind of guys. I started because my primary riding partner is mechanically declined. If I didn't maintain his bike, we would be getting stuck on the side of the road. I won't change the oil in my car, but I love to tinker with my bikes.
Ergo all my friends bikes are adjusted, upgraded and lubed by me. I buy tires 6 at a time now. every spring I have 4 bikes to reshoe. My friend just got a top end tri-bike sent to him by a friend and of course I will have an extra set of wheels to make it all work.
In the last 6 months, I have put 2 saddles, 3 sets of tires, 2 packs of bar tape, and 2 cassettes on friends bikes from my back room supplies. Oh yeah, I finally ran out of the good tubes with the just right stem length.