Every now and then I come across a question that uses American units, which I am not familiar with. That's not a big deal, I simply use Google to convert everything to metric, and it only takes a few minutes.
I also agree with the general consensus here that it should not be a rule to post in any particular unit system, nor to provide conversions yourself when posting. (cf. others system-of-units questions)
However, since in such cases I have done the conversions already, I might as well save a few minutes to the next persons who will read the post, or to myself if I return to it later. Hence I'm always tempted to edit along these lines
the monster is about 5ft tall and exactly 3.14ft wide. Its claws are 5-6 inches long
the monster is about 5ft tall [1.5 m] and exactly 3.14ft wide [0.957072 m]. Its claws are 5-6 inches long. [13-15 cm]
Would that be inappropriate, in the sense that it would be felt as if I'm correcting OP on something that was not required from them in the first place?
EDIT regarding the possible duplicate: two previous questions in system-of-units asked if rules about units existed or were needed, one in general, and the other in the specific case of hard-science questions. The conclusions of both was not to impose any rule and let readers convert units. My question is different because it asks not about rules, but rather if that sort of editing would be considered welcome or rude.