Is it really necessary? Why not just force a wheel into the frame? A few mm can’t make much difference can it? As we say in Yorkshire “it’ll be reet”.
Well I guess anything can be bodged, corners are easy to cut.
As an apprentice I was always taught to either do it right or not bother at all. So in the case of fitting a modern wheel in an old frame, you just need to ask if it is right to need to pull the stays apart and force the wheel in each time you refit your back wheel? If that sounds right to you then go for it.
However to me, a wheel should drop straight in, be in track and be equally spaced between the chainstays and if its one of my own frames, then also equal between the seatstays. And the only way to make sure that happens is to re-track.
Also if the frame is older and has less than 126mm spacing, then it will probably need anew brake bridge and possibly bottom bridge. Otherwise you may end up with unsightly bowed stays. However if your happy with bowed stays, then I guess it doesn’t matter. But why spoil the ship for ha’porth of tar?
Rant over.