Common sense goes a long way here as shown by the suggestions above. The bottom line is that these instruments are expensive and delicate. Do your best to leave it in same condition. Also remember that luthiers/dealers want their guitars to be tested. They didn't travel many miles just for their guitars to sit unplayed. Also put aside any thoughts that your skill level will get you laughed it. If you are that self conscious, you will be more focused on your playing then on hearing the instrument. Loosen up, enjoy your time, and play as many guitars as you want. Would be a terrible shame if you went all the way to miami and just looked at the guitars on display.
With that said, trying these guitars at conventions arn't the most ideal way to test a guitar. The hall can be so noisy and distracting that it becomes difficult to really get a sense of the guitars true sound and nuances, coupled with the fact that you probably arn't playing your own familiar guitar first. If there is an instrument you want to hear more closely, you can ask the luthier if it is possible to test the guitar in a quiet area.
"Success grants its rewards to a few, but is the dream of the multitudes.
Excellence is available to all, but is accepted only by a few." - Christopher Parkening