If it's an event ride or a training/group ride that I've planned with others, it's never too wet! It can be too stormy - you always need to take shelter if lightening is in the area - but not too wet. My road bike tires are Continental GPs - if you do a tire search on this site, you can find a lot of info. They are grippier and less puncture-prone than some other tires, and you really don't want to have just "slicks" on if you are out in the rain. Agree with the advice to lower your tire pressure, avoid painted road lines.... also, be very careful about crossing railroad tracks in the rain: slow down and always cross them at a perpendicular angle - you want to take that angle even when the tracks are dry. Some other thoughts, put things you don't want wet, cell phone for instance, in zip-locs or other waterproof bags. I swear by aLOKSAK bags, and love that you can use your touch screen without taking the phone out of the bag. Carry a plastic grocery/Target-style bag and a few rubber bands to put over your seat when you are not riding. Happy riding!