Dear Carl,
After Sedgewick, I clean it whenever there is debris in the tub. (I check after every use.) I think the risk of ignition is too high with chaff and oil and fuel in the tub. I hand pick out all the big stuff, then blow out what I can and move to water. I always use detergent (dawn or simple green) on all oil spots in the tub after an oil change. That way, if I see a dirt patch, I know I have an oil weep from something and I can get on top of it while it is still just weeping, not spraying.
I powerwash it about every other month, or whenever I think I won't be able to see the weeps. Like MR, I get all of the solids out first, then use detergent and water. I haven't actively washed the tunnel, so I think I'll have a look see to figure out what I have been missing.
Love the ziplock idea; it sounds better than the little plastic tray that I've been using.
All the best,
Peter