Note that the IMD reading is only accurate if the PSK31 signal is idling and the strength is well above the noise level.
Exactly, which is why I don't bother with reporting IMD in most cases. S/N ratio is a more useful figure, but even that doesn't necessarily tell the SENDER that much information, but rather is indicative of the combination of "ingredients" that make up the signal path as observed from the receiver's viewpoint; from local noise to atmospheric noise to ionospheric conditions to band selection to band occupancy to bandwidth selection to transmit power to transmit antenna to receive antenna to sound card quality to ........
As someone pointed out, in the end it doesn't really matter whether you use RST or RSQ or something else, as many ops are just looking for that 599 and don't care about anything else other than making the contact. It is simply a matter of personal choice on your part and so do what you feel comfortable with.