should it rain on my compost heap?

:?:
 I have built up a heap with layers of grass cuttings/cow poo/horse poo/upside down turf and potato foliage etc.
I usually keep it covered but should I let it get rained on sometimes :?:

keep it damp jim but not wet or dry.
hows the comfrey dispenser working is it producing the goods yet ?

Thanks Shaun.
I looked yesterday - nothing yet - its only been in about a week.
Remember I put a cap on the top to keep the rain out.
Do you think I should give it a little water?