Paul,
From the outset, I decided that if others could do better work on my car than I could, and the price was reasonable, then I'd let them at it. While I might not be able to say that I did it all with my own two hands, at least everything should be done right the first time.
Wiring was a case in point. For the thousands of dollars that I had invested in the M800 Motec, multiple temp sensors, hall effect sensors, magnetic pickup sensors, multiple pressure sensors, MSD Digital 6, HVC coil, gauges and senders, there was no way that I felt comfortable with risking it all to save $900- for a custom engine loom made from MilSpec wires, correct shielded cable, Raychem heatshield, gold pins for the M800, and all wires labelled. This runs through a MilSpec Connector at the firewall so that with a 1/4 twist of the connector, my entire engine loom can be taken from the car still attached to the engine. It's interesting to note that no MilSpec wires that I've ever seen have been soldered. They are all crimped using a $$$pecial crimping tool.
I might have done some study and gone ahead and done the rest of the car, as that all seems a lot less important to me than the ECU harness. To be honest, for the extra money I couldn't see why not to let the one guy do the whole car. That way, if there are ever any wiring problems, the car goes back to him and it's all fixed!
Here's a 100K photo of most of my wiring:
http://www.home.gil.com.au/~marsh/motecwiring.jpg