A few years ago, I'd decide whether or not to go out by looking at the hourly forecasts. I found that a) the rain often didn't show up, b) when it did, it usually wasn't that bad and c) I needlessly missed too many riding opportunities during a period when ride time wasn't always easy to come by.
Since then, I'll generally ride on my usual days unless it's already coming down pretty good at the outset; I don't mind getting caught in the rain, but I don't like to set out in anything worse than light rain.
Oh, and I don't ride if there's a credible threat of severe weather, though I'll sometimes squeeze in a quick 20-miler if I think that I can make it back home before it gets too hairy.