Yes, this is probably true. As you play a new guitar (or other wooden instrument), the fibers in the wood settle somewhat due to the vibration, and over time this causes the wood to become stiffer, more stable, and more resonant, which in turn improves the sound.
Different woods experience this phenomenon differently; for example, spruce takes about a year of playing to break in, and a guitar with a spruce top will sound better after a year of playing than it did right out of the box. After that, the aging process is slower and the marginal improvements smaller. Cedar, on the other hand, breaks in both sooner and more steadily---you may notice a slight improvement in resonance earlier than you would with a spruce top, but the overall improvement after a year may not be as much as with spruce.
All of this is subjective, of course, but it reflects a lot of people's experience with wood instruments.