It's my experience that pretty much every vintage steel frame is bent. They are fairly large and lightweight tubular structures that are used very hard. By the time they are thirty or forty years old they aren't that straight. If you do something crazy like riding them instead of looking at them they will get progressively less and less straight. Every time I have a frame aligned it rides a great deal better than it did before. Aligned as in on a frame table by a framebuilder. Or you can straighten them to 'good enough', as in good enough that the gears shift, the chain doesn't fly off, and it doesn't pull too hard to the side. Or you can ride them as is and then discuss the fine points of frame geometry.

It's just like tuning a piano. You keep doing it. A good pianist can play an out of tune piano and make great music. For anyone who uses the piano hard the instrument is noticeably further from tuning at the end of the performance than at the beginning. So you call the piano tuner back. Or the instrument slides downhill to junk.