Yes, audio reports are better done on simplex. However, if a request is made, quite a bit of the time, it is just to see whether the audio is coming through clearly and if it is easily understandable. Most repeaters won't modify audio so much as to bother the person asking for the report.
It comes down to if the person requesting wants to know particulars--whether or not too much base is present, if the midrange is sufficient, or if there is too much tinnyness coming through. THOSE reports should be done on simplex, but for the simple "Am I coming through clearly" report, it makes little difference. That is, unless you KNOW that the repeater being used has been adjusted to an extreme.