[dTal]

How are you getting a measurement of 51.95mm from a ruler with 1mm ticks? If your ruler is capable of outputting that, and if those digits are actually meaningful (even if they do have error bars) then obviously yes they are definitionally significant digits and you shouldn't round them away.

[devereaux]

Good point. Maybe not 51.95, but 51.9 mm you can say if you eyeball the measure to be quite close to the mm mark, but not exactly there.

Repeat enough time and you can interpolate 51.9, which you wouldn't have been if you had thrown away the precise measurement even if the precision is within your measurement errors (of 1 mm here)

[godelski]

Your smallest tick is 1mm, not 0.1mm. You can't measure to 51.9mm. So your measurement is 52mm.