I do clean the fridge every now and again, but I admit that it's been awhile. The last time I "cleaned" it, I just removed all of the old leftovers that had grown science projects. I didn't wipe down the shelves or anything else that required more than a quick minute.
The best time to clean the refrigerator is at a low point in the grocery cycle. I'm definitely there.
I threw quite a few things away, wiped down all the shelves and washed the crisper drawers. I no longer have containers with mystery contents hanging out in the back of the fridge.