CCL: when to take a logarithm of biological activity in QSAR studies?

Dear CCLers,
 there is an established tradition of taking a LOGARITHM of concentrations
 (e.g. -log (IC50), -log(ED50) ) as biological activity parameter in QSAR
 studies. As I understand, this comes either from the need to bring widely
 different numerical data (like 0.0001 and 0.1) into more balanced scale,
 or from generally logarithmic nature of biological response to external
 stimuli. Now, my question is: do you think that when applying other,
 non-concetration-based types of biological activity, one should also take
 their logarithm for QSAR studies? I am thinking specifically about
 percentages, as in "inhibition percentage", which are commonly
 as results of biological assays.
 Happy New Year to everybody!
 Jaroslaw Panek
 Faculty of Chemistry, University of Wroclaw
 ul. F. Joliot-Curie 14, 50-383 Wroclaw, Poland