I've thought some about this. Using the fitting on the deck really sprays the water around, and is pretty useless unless you have the blades spinning, in which case it's still pretty useless.
This leaves just using a regular hose and spray tip. So, you've finished mowing, want to clean the deck. Option "A", leave the deck in place, engine off, reach under with a stick or some such and pry the bulk of the junk off, then hose out the remainder. By the time you get to the water part, the deck has cooled somewhat, and the bearings, if they have any grease in them at all, shouldn't be but just barely too hot to touch while running, and have had time by now to transfer a good bit of their heat to the housing and to the air. So hitting them with 50-60 degree water shouldn't hurt anything.
Option "B", removing the deck, takes a bit longer, the bearings have had even more time to cool, so even less likelihood of hurting them.
The danger of spraying at the bearings directly comes in water pressure pushing water past the seals into the bearings themselves. If they are full of grease, there's not much room for water, and frequent use will heat them up and drive out whatever was there. This is if you're using a regular sray from a garden hose. If you use a pressure washer, you can really drive water into the bearing and even displace the grease to some extent, and if you don't use the deck for a while, the bearings can begin to rust. Even microscopic amounts of corrosion (rust) will ruin the balls or rollers and pit the races, speeding up the demise of the bearing.
You have to really aim hard at the seal to do this, and the sheave on one side, and the little cap on the blade side can usually prevent water from getting past the seal because they deflect the spray.
Worst case scenario is poorly greased bearings that are already overheated, getting sprayed immediately with very cold water. But it's more the overheating of the bearing that has damaged it.
This is my opinion, others may vary. I routinely spray my decks fairly soon after mowing, but avoid a direct shot on the bearing housing. I also keep the bearings well greased (a good shot after washing will help push out any water that may find its way in). I am still on original bearings on the 48C deck on my GT235 with over 435 hrs, and expect them to go waaaaaay past 1000 before anything undue occurs.