I generally clean my decks weekly, just use the mojack, lift and use a plastic scraper then the air hose.
ProLikeJesus,
I see there being two potential "problems" from accumulated grass/dirt/etc. under a mower deck. The first has been eluded to and that is trapped moisture causing rust and other deck material damage. The second(from my experience) is enough accumulated buildup that it "restricts" the ability of the blades to turn freely after hardening between mowings and when you go to mow again and engage the PTO.....ooooops there went my drive belt(if the blade won't turn.......something's gotta give:0)
In my last two lawn tractors(most recently a Craftsman 54"...made by Husqvarna) the stamped deck design and the blade tip speed allowed quite a bit of accumulation under the deck and after throwing one belt I began spraying underneath the deck with a garden hose each time I mowed to remove most of the large clumps to avoid that happening again.
You may notice some of the recent models of residential lawn tractors(and some ZTR's) have that hose-attachment dooie that allows you to attach a garden hose, turn it on, engage the blades and wash the deck out. My Craftsman didn't have that and I thought I wanted that on my next mower. What I discovered when shopping for a ZTR, though, was that a good quality ZTR(especially the Commercial units) don't suffer as much buildup due to (a)the deck design, particularly the fabricated decks with baffles underneath and (b)the blade tip speeds of around 18,000 fpm(there is so much turbulence and clipping ejection the stuff just can't "stick to" the deck)!!! I have mowed my yard 4 times with my new Gravely Pro-Turn 52" Commercial ZTR since buying it(this weekend after a rain the previous day) and upon inspecting under the deck each time about all I see is a "green stain" on the deck and spindles and very little if any grass stuck to the deck surface underneath(making me a VERY happy post-mowing cleanup guy)!!! I, like user Ric have purchased a MoJack and I will probably clean under my deck like he has recommended occaissionally(every 5 or 6 mowings).
I've also heard and had some advice about spraying underneath a new mower's deck with PAM cooking oil spay and every few mowings but I can't recommend that from experience. I actually tried a product called ZEPLON when I first bought the Craftsman lawn tractor(a "hard drying" form of silicone) but it didn't help a bit. Personally, I don't understand why some mower manufacturer hasn't come up with a mower deck that has a "teflon coating" underneath it.....as they would sell like hotcakes:0)