For accuracy you can't rely on the mains frequency being exactly 50Hz (or 60Hz depending on where you live). There's normally a tolerance band around that. In the UK there's a statutory band of 49.5Hz-50.5Hz with operational ones set by the National Grid (the primary power provider) of 49.8Hz-50.2Hz. I think Australia did have a tighter nominal tolerance of 49.9Hz-50.1Hz but this has been relaxed. But even then, sudden changes in load (e.g. TV commercial time when a large number of people put kettles on) or a power station going off-line will cause bigger variations in frequency.

And it now seems that there's less concern about absolute frequency (which will always be fairly close to the nominal) and more about maintaining a tight phase angle between the different mains phases.

A 50.5Hz mains would give A=440Hz as A=444.4Hz, or 49.5Hz mains would give A=440Hz as A=435.6Hz. So you could be quite a long way out.

For accurate tuning of a disk-type strobe, you are better off using a speed controlled motor with a crystal clock plus disk speed feedback for extra stability. Conn strobe tuners used to use a mains-synchronised motor but switched to a crystal clock after they discovered the real-world variability of the mains frequency. If everyone used the same tuner, then it wouldn't matter quite so much, but if some were using battery ones, and others tuning forks, then any frequency drift would be quite noticeable.