Rain and teh newbie....
 

I'm still at <5 races total.

The Tour of Washington (Missouri) is scheduled for this Sunday.

I'm registered, but the forecast is called for rain.

As a relative newb, should I just grin and bear it and run in the rain for the experience, or stay home and avoid the risk of getting swept up in a wreck?

One of my first races was at the Central Valley Classic last year where we did the RR in a complete downpour. It was actually worse than that because halfway through we started getting hail !
The RR was completely flat with wide open roads which made things a bit easier. What is the course like for your race? If the roads don't involve insanely twisty descents on bad pavement or roads with potholes, etc. then I would still go. Who knows? Maybe you will have a good time and do well. At the very least it will count towards your CAT5-4 upgrade. I'd prepare accordingly if you do go and get one of those clear rain jackets so you don't get soaked.

The course is a "4.8 mile loop through rolling terrain."

If it's rolling terrain I'd go do it. If the conditions are really bad (torrential downpour) you can always separate from the pack. Sometimes rollers can separate the pack anyways.

Nothing wrong with rain. Unless it's predicted to be really, really cold, go for it.

Jump early. Everyone will be really skittish. If its really bad just go out and to an ITT.