As far as I know...
...there is no official rule regarding what unit system to use
Aside from that it is usually silently agreed upon, as you also say, to use the unit system the OP uses to describe the problem. If they use meters, use meters; if they use feet, do the conversion (which isn't that hard to be fair).
* When Op didn't use any unit system in his question:
If they used numbers without annotating them with units (aka meaning) it's best to point it out to them via commentary and/or propose an edit that adds units to all numbers (this might or might not be met with goodwill by the OP, but is a legit practice as you merely strive to improve quality; you do not have a right to force them to accept said edit though..)
If they do not make use of any numbers/units in their question but it is clearly their intent to get numbers as a result, just use what you prefer. You cannot know if they understand metric/imperial before they say so themselves.
How I tend to see it:
The Stackexchange network and its users are all over the world; the Network uses UTC instead of anything localized and thus you should also strive for using SI units whenever you can.