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.