The more the merrier! If there is an aesthetic problem or it becomes an attractant, I bury it within the first 4 inches (or even deeper).

When experimenting with worm enclosed boxes and additives I find the coffee grounds bring in mites and the worms start going away. It changed the pH within the container. This showed me how important diversity is. But that's in a box, not in the garden. If I have too much of one ingredient of kitchen scraps I try to combine it with other things or spread it out over a wider area to thin it out.

Most times I just toss it and forget about it!

According to what I can find on the web it says that worms like coffee grounds as long as they are not over whelmed with them. It also says that coffee grounds are a ant, snail & some other insect repellent. I can tell that my vegetables are growing better than they used to, it seems to have helped the soil.

Well, that's all that matters. The plants tell all!

I had a bed of fall brassica that looked horrible and was being attacked by caterpillars. I spread coffee grounds on top of the dirt and poured a milk and mollasses mixture on it. While it also got some organic fertilizer, I think the coffee grounds, milk and mollasses helped with the soil microbes. The bed took off. In fact, I had put some replacement plants in thinking the others would die, so of course they are a bit crowded now. I usually compost my coffee grounds, but DH is revising the composting, so I had a bunch to put out.

The only problem I have ever heard was that they can repel water if they are solid grounds (nothing else mixed in) and too thick.

Coffee grounds should be scattered and worked in to the soil or sprinkled thin enough to be watered in never just dumped in a pile on the ground.