I've done maybe half a dozen, but prefer not to if at all possible.

Personally, I'd rather just run the wheel in there that the frame was designed for, and
have, in the past with used bikes that came to me with rear hub and frame widths
mismatched, just built up the appropriate wheels.

But it is, as stated by others here, easily done with the proper technique.

I clamp the bare BB shell in a large bench vise on a solid bench attached to the wall
with the BB shell protected by wood or soft jaws from the vise. Set up your magic
measuring string from the same point on one rear dropout to it's fellow on the other side,
going around the head tube, and mark two spots to measure from string to seat tube
on the tube itself, which should be opposite and at the closest point of tube to string.

Measure with the depth gauge of a vernier caliper......even the two dollar plastic ones
from Harbor Freight work well enough.


We have one of those Park frame bending thingies at the coop, but it's not hard to
improvise something using scrap lumber.



I don't get the Sheldon Brown method, as it seems to me very problematic to
measure your progress on each side as you go along.



Interestingly enough, if you buy a lot of used bikes or work on them, they alignment
is very often off when you get them, so in my opinion, those are fair game for setting
them at whatever width you prefer as you straighten them.