Please don't.  We understand the excitement of being close to a celebrity - especially an artist whose music moves you.  But, the coaches are working.  Even when it looks like nothing is happening, they are listening to producers, thinking about what's best for their teams, etc.  There really isn't time for them to sign a bunch of autographs, and even though you think "I'm just one person" - there are hundreds of "just one persons" in the audience.