Yes... and no. I've not done so in a garage, but several in my houses. It was pretty disheartening to see a TV show where people who thought they were savvy allowed their house to be broken into by pros. The burglars found their "secret panel" inside their closet in about 5 minutes.
It's a good idea, but to have any chance of success, it has to be at least as strong as the surrounding areas, whatever they are. In the case I cite, the theives dashed into several closets, saying "People try to hide things in here" and just beat on the walls with their fists. Hidden panels popped right open.
The other problem is unless you live in a castle, the extra space must be small or it will be noticeable. A closet that is shallower than it should be will be suspicious.
And finally, remember that unlike you, a thief isn't concerned with being neat. He'll just rip down an entire suspended ceiling to see if you've hidden anything above it.
Good luck!
(and if you don't need it very often, put it in a safe deposit box at your bank)