i know what the data sheet says but:  
The only time i ever need to replace the regs is when they start to drop 1 or 2 volts with just connecting a 555 lamp as a load. or because they are just dead ( reason why i use a 555 lamp is because they are always easy to get a hold of when working on a machine, i haven't tested what current they draw but if the reg should power one of these without any problems at all. I will always replace C4 first and try the test again before replacing the reg)

once you have replaced / fixed the connector issues the next major problem i have is with C4 on the driver board and C31 on the cpu board. WMS used a few different suppliers for these caps but if you have a dark blue coloured capacitor fitted it will be either stuffed or on it's way out. I grabbed 2 random boards out of my repair pile and cut one lead on the cap and hooked them to a capacitance meter (see photos) to prove this.
they should read around 100uf but one is down to 73uf and the other 62uf.
If you have a grey or beige coloured cap they are usually ok and very close to 100 uf

the next problem you may encounter is a faulty cap and or bridge before the regulator

The reason why Brett mentioned the regulator is probably because I borrowed a few off him and the only time he heard of the faults i was repairing was when i asked for a couple of regs so he must have assumed that they were always the culprit.

I always work with the idea is there are so many of them made and neither bally or williams put an adjusting pot in any of there 5 volt regulated circuits from the first solid state machine to the last dmd machine and even after 30 years a large number of them are still running fine without an adjuster. so instead of cutting traces and fitting jumper wires to a pot (which i have seen done on several occasions by one other repairer who leaves the faulty caps in place even after he has done this mod and it just looks like one big hack job) I will replace a few faulty components and get it working like all the other pinball power supplies out there.

i agree with you mike on the "For the original designers to think that there would be ZERO resistance in all of the PCB tracks, connectors, crimps and cables between the 5V reg and the micro reset circuit was madness! I'm surprised they don't give a LOT more trouble."
but there is also circuitry that runs off the 5 volt reg on the power driver board (WPC) and if some one tried to compensate the lack of voltage getting to the cpu board by turning up the voltage then they could damage components on the driver board.

on my getaway which is a wpc machine with good connectors the drop between C4 on the driver board and C31 on the cpu is only 0.1 volts in attract mode , but this has had boards rebuilt by Beaky fitted