Originally posted by Matt Rowe:
You might also consider that there are rain tires for heavy water, and intermediate tires for when the track is wet but standing water is minimal.
If there's no standing water, I've always been faster on normal slicks. The line dries up after 2-3 laps anyway.

Of course, sometimes it's nice to have some partial tread tires to run when the track is a bit damp and more rain may be on it's way.