Most people go by factory defaults settings, which were 5 balls for older EMs, and 3 balls for newer solid state and DMD machines.

I had all my DMD machines set for 5 ball for a while, until I got real good at them, then I erased the high scores and reset everything to 3 ball, and I've been having good fun with that. I think 3 ball play is more what the designers had in mind for the DMD machines, and with all my Williams/Bally machines, I've been able to get to the wizard modes in 3 balls, although it does take some good play to do it.

Playing 3 balls on the newer machines also allows me to compare my scores more easily with people here, as almost everyone plays 3 balls on the newer machines.

Now, if I had LOTR or TSPP in my collection, I must admit I'd be tempted to try them on 5 for a while, just to see if I could get to the wizard modes, as their rulesets are so deep, I don't know if I could ever do it on 3 ball.