ECU 5v supply fluctuation

The few I've checked have almost no variation. One thing that may mislead you is the earth, are you measuring between the car body or battery negative and the TPS supply? The TPS signal to the ECU is not usually referenced to earth directly, it's another wire to the TPS that goes direct to the TPS and if it's earthed then it's by an indirect route probably through the ECU, so you could get misleading readings if you don't measure directly across 2 terminals on the TPS. Your 0.5v is very high though so you my have a bad connection somewhere. Some Nissans even respond well performance-wise to directly earthing the 'earthy' TPS terminal with an extra wire to cut down this extraneous voltage.

I was measuring between the TPS and battery negative, so I tried again using the ground at the TPS, and got the same thing. The voltage ranged from about 5.2 to about 5.8. It was most stable at idle, but even then was probably wavering over a range of about .3 volts. If this isn't normal, any idea what might cause this?

I suspect that since I get the same thing whether I'm using the ground at the TPS or at the battery, it's not an issue of a bad ground.. is there a regulator built into the ECU that provides it with a good +5v? From what I can tell the ECU is only fed 12v. I don't think there is any spec in the service manual as to any tolerances for the 5v supply.

I think you'll have to measure another car of the same year and type to be sure. It's normal by the way to measure the voltages without the engine running, just have the key turned so the ignition is on.

The 5V source is derived from a solid state voltage regulator. These type of regulators are extremely stable and accurate. Your 5V should certainly be between 4.95 and 5.05 volts.