Some people recommend deflating your tires by a few PSI when riding in the rain. The argument is that you can get better traction on slick wet pavement.
Others say that if you have a good tire, it should work well in the rain, and that lowering the PSI will only make you more susceptible to flats.
Personally, I like to lower my tire pressure just a tad, but the main reason isn’t for better traction. It’s so that I don’t ride as fast (insert slow joke here), which in turn makes me feel a little safer.
Whatever your method, ride smart and safe!