Depends on what you can handle - exact conditions when it does come down, temps, wind etc.

I've ridden with/without rain gear in the rain and it is always a guess whether I'm going to be uncomfortable or not, too cold or too warm. Some rain jackets(Performance has some) have enough vents, at the sides for example to provide some cooling.

If using a car to get to my ride and if rain is likely, I always carry a jacket in my jersey pocket, tights too. This mainly so I can peel them off and remain relatively clean in the car!