The one and only time I've been to a Guitar Center, I was trying out "high end" acoustics. I asked a salesman for help and he showed me into a special room where they kept the good ones. He then left me alone to try any models I wanted. There were several other musicians in there and we felt free to pass the guitars around, try others, and hang them back up.
Music shops...and Guitar Center in particular are used to seeing young people come in and paw all over their merchandise. This must make store owners nervous. You can demonstrate your responsibility by approaching a salesperson and asking permission to try some guitars. Then take your cues from him or her. My experience in other shops is that if the guitar is easy to hang back up, you should do it. If it's too high on the wall to safely replace without a ladder, leave it for the salesman...but let them know you are done and the guitar is ready to hang back up.
I'm a middle aged adult, so it's probable that salespeople feel comfortable giving me more space than if I was a teen. As with all types of etiquette, err on the side of caution and be courteous. If you act like a teen showoff who just wants to come in and hammer out loud chords on their floor models, don't expect to get the same treatment as a responsible adult / potential buyer. Here's what you should do:
1. Ask permission. Tell the salesperson what you are interested in, and that you'd like to try several guitars out for comparison.
2. The salesperson will probably get the first guitar for you, and help you plug it in to an amp. Take your cues from there. If they hover over you, you'll know they're not comfortable with you self-serving. Handle the guitar better than you would your own. If you demonstrate that you aren't going to wreck any of the merchandise, the salespeople are more likely to give you some space.
3. When you're finished with the first guitar, if the salesperson has wandered off, hang the guitar back up if you can safely do so. If you can easily get the other guitar you want to try, get it yourself. If not, approach a salesperson.
4. When you are done, it's a matter of courtesy to thank the salesperson and let them know if you're interested in those guitars. After all, that's why you came in...to try and potentially buy a guitar right?
Just by asking the questions here, you've shown that you know how to be courteous and responsible...an increasingly rare quality. What you're asking really is whether it's ok to suspend that etiquette at a Guitar Center. I think in your heart, you know the answer.