I think that, as wood and glue dries out, the guitar will change its tone a bit whether it is played or not. I can't think of any reason why it would change simply from being played; wood that has been vibrated for a long period of time won't change its atomic makeup or anything.
I believe there are two other factors that play a larger role in the changing sound of a guitar as it ages:
1 - simply playing. The more you play, the better you get, the more technique you add to the arsenal, the more musical numbers, the more varied you become with your musical selections, etc...
2 - The more you learn about your guitar, the better you become at things like string selection that makes a huge difference.
Speaking of my own experience, I love my all hog Larrivee OM-03 vintage. It is a thing of beauty and always puts a smile on my face. I think it's gotten a bit louder and expressive since I picked it up a year and a half ago, but I'm also a better player than the day I brought it home. I've learned that it sounds better when I grow my nails out than when I play with flesh (my L-01 with spruce top is the opposite) and I've learned just recently that 80/20 strings make it really come to life.
Bottom line, does the guitar sound different as it ages because of the wood? Maybe a bit, but I think the person playing it changes much faster.
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