I have been mulching my garden with fresh grass clippings for over 15
years and never had any problems whatsoever. When I first put them down
in the spring, I use about a 4"- 8" layer around everything. By the end
of the day, or the end of the next (depending on what time I mow,) the
clippings have dried out and the layer has reduced to about half of what
it was when fresh. They do not decompose quickly enough to release
enough nitrogen to cause burning problems. As the season progresses, I
just replace what has decomposed. I have even used them when they were
full of weed seeds, such as dandelion, and haven't had any weed
problems. They mat down as they dry and hold water in nicely. As far as
pests using the clippings for cover or clippings causing diseases, I
have not had a problem with either. In my opinion, grass clippings are
the perfect mulch...with one caveat:

When using grass clippings is that you don't want to use them from a
lawn that has weed killer used on it. The grass will pick up the
chemicals and as the clippings decompose in your garden, the chemicals
will be left. It would be better to compost clippings that come from a
non organic yard so that the chemicals have the chance to break down
before being used in your garden.

All that said, I have found that using grass clippings as a mulch varies
from gardener to gardener, as the number of postings show. I say try it
out on your garden to see if it will work for you. If you have concerns
about any damage or pest problems they might cause, experiment in a
small section of your garden. After all, isn't experimenting what
gardening is all about?