Guitars are very subjective. I don't like it when sales people prey on people's ignorance to sell a guitar. I feel that this is what is happening.
Here is my first problem with this assertion. The idea that a guitar will sound better after you play for a while is not provable. It's difficult enough to figure out what you like with a rack of guitars that are right in front of you. Trying to figure out how a guitar compares to what it sounded like 6 months ago is virtually impossible. Marketing is split between tangible features and intangible. Perception of sound is mostly intangible. That is why you see a lot of focus on the best sound with higher end guitars. If you can't tell the difference, do you want to rely on the salesperson to tell you how much to spend?
Second, there are a million little things that people focus on to get the "best" sound. There are big things and there are little things. Focus on the big things first.
- Focus on you and what you like. Spend some time figuring out what sound you are trying to achieve. You have to play a lot of guitars to discover this. Know for yourself and don't believe the person trying to sell you something.
- Look at your amp. Even the best guitar will sound like crap with a poor amp.
- Look at your pickups. Different pickups sound different: dark, bright, crisp, noisy, noiseless, etc.
After you have spent some serious time in these things, then consider pickup placement on different guitars. All these things make a lot more difference than "playing the guitar for 6 months."
I am not trying to dodge the question. I am just saying that it is not important when compared to these other things. Maybe there is something that changes the sound over time (vibration, moisture, moonlight, etc.), but who can legitimately confirm that this will make it sound better? Maybe it will sound worse. If guitars sound better after they have been played, then buy used. Again, it just sounds like marketing. I recommend focusing on whether you are happy with the guitar today, not possibly some time in the future.
I hope this helps. ;)