you'll prolly get varying answers due to personal preferences, but outside of scraping the deck underside with a nonmetallic tool or running the deck thru dry leaves to clean out grass, some coating concoctions will work at least for a bit.
Pam should not hurt your deck if applied to a cleaned surface
and should at least temporarily help in preventing grass sticking. But, more moist grass is will cause it to stick more easily. And if you leave stuck grass there and paint was already partially worn off, then rusting may set in.

Anything that is sprayed below (mo-deck, pam, silicone, wd 40 or even wd 45 will help for a cutting or two and then you are back to the same thing you started with. They may help you if you are cleaning thoroughly each time however.

As a rule of thumb I clean my deck in the middle of the mowing season and now will perform the same task at the end, for storage. I have used tranny fluid and it worked ok, last time I sprayed fluid film on it. The deck will get pulled tomorrow we will see. The easiest way to semi clean it is chop leaves this process is very effective.It just cuts down on the sraping .My feeling is the leaves did more than the fluid film.

Yep. That time of the year, pulled the deck and put on the blade, weights and chains. Took off the blades, sharpened and balanced them and did the scraping and wire brushing of the deck. Usually just pick up some cheap "rattle can" paint and put on a few coats a few days apart. Have tried the rustoleum etc. or more expensive paint but whatever goes on is essentially blasted off anyway.

Been reading about auto undercoating with silicon on other forums. It's available at the local TSC for $5.99 a can and should be at least as good as the paint. I wonder if it goes on as smooth as paint or "clumps" and would thereby give grass a place to catch hold?

Anyone tried it?

The verdict is in , pulled the deck and it was clean . The leaves did their job and the fluid film helped.

This comes up from time to time. I agree the only solution would be to clean it. grass has moisture in it and especially if you over sandy soil mositure and dirt eventually starts sticking to the deck requardless what to try. I pull mine at least every 6 weeks or after I have to mow in after rain.

Try spraying the lawn with PAM. Or was that roundup?