Newbie needs help ...

Depends on the car, the build, the use, the tire on that wheel, etc.

On a 67-69 dart I would not go 20" either but to blancket all cars older cars like that, no.

Also, while maybe 20's aren't he ideal look for many, i've had classic cars come with 13's and 14's. Going up some from there with a decent aspect ratio yields a nice look and good function.
While I agree it can work on some older cars I’ll stand by my opinion for most. Cars are designed with a given wheel size in mind. The size of the wheel wells is meant to complement and work with that wheel. Put a twenty inch wheel on a car designed for fourteen or fifteen and it usually doesn’t work out any better than it did for Jenee’s car. Ride height is also messed up by twenty inch wheels and if something isn’t done to compensate as described in Cuda416’s post above the poor car ends up looking like it’s ready to go off-roading. No thanks. Of course this is just my opinion, and you know what they say about opinions.