I wouldn't go so far to say that it's unethical. It may not be the best time to hunt, but then again, it may actually be some of the best times to hunt. With rain, you know it'll wash away a blood trail rather quickly so it would be unethical, and unfair to the deer (or whatever you'll be shooting)not to give it your absolute best shot that you can make. But then again, that goes for perfect weather as well.

If I know I've hit a deer with the "perfect" shot, that being a double lung, I'll give him about 5 minutes and go after him very quietly. I've yet to double lung a deer and have him live for more than a couple of minutes. I'm talking about a solid double lung shot. I'm not talking about hitting one lung and knicking the other.