I slow down in porportion to how much it's raining, and how stupid traffic is being at the moment. If it's just sprinking and noone's freaking out, I won't slow down signifigantly. If it's so bad that I can't see the lines but I can see water flowing down the freeway, you know I'm slowing down. If it's moderately raining and everyone is doing that "bumper to bumper so they can get home before getting wet" thing, I'm in the right lane doing my thing. Given that I'm usually one of the faster drivers, when I slow down often doesn't put me excessively slow, it just means that I stay out of the left lane most of the time.