Arn Butt Bill said it best. There is no definite answer. There are some roads that are unsafe to go straight on when its raining, but some of the newer, better surfaces grip quite well in the rain.
A little tip that I learned from riding with yellow_wolf is when riding in the rain just slide your foot on the pavement for a few feet. This will give you a very good idea of the available traction. Sometimes after doing this quick check for traction I have drug a peg in the rain, sometimes it scares the crap outta me to go straight at speed. I know, I know..... there is always the risk of the unseen like fuel spills, mud stuff like that, but hey, thats the chances you take riding in the rain.
FWIW; The new highway bill recently signed into law has new and improved mandated guidelines for skid rating on all road surfaces. This is great for improved traction, but it is gonna suck for tire life. More traction equal more tire wear.