I think it was normal for mechanical shutters to consider 20% mistake an acceptable one. That would be a bit less than 1/4 EV. When buying second hand, the best would be buying from those repairers who make a business of buying broken old cameras and repair and cannibalise them.
I bought a Nikon FE2 second-hand from such one repairer on eBay. He gave me the sheet of all actual shutter speeds. Seeing how serious his work was, I also brought him four Minolta bodies (SRT 100x, SRT 101b, X-700 and XM) and he did the cleaning and gave me a sheet with actual speed for each.
When properly serviced, discrepancies are quite contained. I am surprised by the X-700 which at 1/1000 (maximum speed) overexposes by 36%. (1/3 EV I presume). The second worse time is 1/500, 9% mistake.
The Minolta XM (bought very old and used, probably by a professional) has the maximum mistakes at 1/500 (15%) and 1/1000 (14%). That's a very small mistake anyway, probably 1/6 of an EV.
My Nikon FE2 has the maximum mistakes in 1/250 (11,25%) and 1/4000 (10%).
My Minolta SRT 100x has the maximum mistakes at 1/30 (23%) and 1/8 (12%). Curiously, the fast times (1/1000, 1/500, 1/250) are extremely accurate (mistakes resp. 1,5%, 0,5%, 3%).
My Minolta SRT 101b has the worst three times at 1/30, 1/8, 1/15 (resp. 20%, 20%, 11,6%). Again fast times very accurate.
The tests were carried on with an oscilloscope by a person who has a laboratory for medical appliances maintenance.
General conclusion: A mistake within 20% is generally speaking quite acceptable, you are not going to suffer it at all shutter speeds.
Diaphragms and film speed also introduce discrepancies between theoretical exposure and final effect.
As far as I know, modern film, also "unprofessional" one, is produced to a 1/6 EV tolerance or so. Your shutter might give you a mistake of another 1/6 EV and your diaphragm maybe the same. So if you are unlucky, and all three mistakes have the same "direction", you end up with 1/2 EV, which is certainly noticeable on a slide.
Nonetheless, using auto exposure certainly can cause a much higher mistake
Fabrizio
EDIT So if one uses negative film, exposing for theoretical ISO will place the mistake by a slow central shutter in the "safety zone" and all is fine. If one uses slide film, I would keep in mind the real times where there is a big discrepancy and have also the diaphragm checked.