I use real dirt for the most part as well, the soil here in AZ is very fine and using gold mining sieves (5 gallon bucket size) makes classifying it into different grades and ballast and such simple. On my current layout I have no use for "garden soil" and such, otherwise I would still consider using coffee grounds. I used it for years on my childhood layout, and never saw any signs of mold (this in MI, where it is just a wee bit on the humid side). If you have mold growing on your layout, I think the massive amount of moisture present to make mold grow (on anything) is approximately 1,000x the problem that having coffee grounds on your layout could ever be.
Also: The replies I see saying that things will grow on coffee grounds on your layout all seem to be from people who have no experience with the matter.
