"But until we actually know what smaller-scale structure".

From http://en.wikipedia.org/wiki/Planck_Length: "Combined, these two theories imply that it is impossible to measure position to a precision greater than the Planck length, or duration to a precision greater than the time a photon traveling at c would take to travel a Planck length"

Therefore, one could in fact say that all time- and distance- derived measurements can in fact be truncated to a fixed number of decimal places without losing any real precision, by using precisions based on the Planck Length. There's no point in having precision smaller than the limits in the quote above, as anything smaller is unobservable in our current understanding of physics.

That length is approximately 1.6 x 10^-35, and the corresponding time duration is approximately 5.33702552 x 10^-44 seconds.