FAR Part 97: Aircraft Approach Categories - IAS vs Ground Speed

I'm an Instrument Airplane student, and am having a disagreement
with my instructor on one topic - that of Aircraft Approach Categories.

According to 14 CFR Part 97.3 (b), it provides the speed ranges for
the different aircraft categories (A-E). In all the documentation I've
read, this "speed" is the IAS of the airplane.

However, my CFI says that this is based on the Ground Speed. When
we are flying an approach with a tail wind and can see that, although
we are remaining below 90kts IAS, our Ground Speed (shown by the GPS unit)
is just over 90kts, he said I must use the category B minimums.

I understand his reasoning (in that the faster we're moving across
the ground, the faster we'll move out of the protection zone, etc.),
but from what I can find, the FAR doesn't mention ground speed at all.

If I use the minimums associated with the higher of the IAS or
Ground Speed, would I get dinged during a proficiency check?

The reason I ask is that I've been asked questions before where the
examiner was trying to make sure that I completely understood the
rule, and I'm worried that selecting minimums that are higher than
necessary will show that I don't really understand it.

Thanks,

--
Mark Hansen, PP-ASEL, Instrument Student
Sacramento, CA