15 minutes is a long time to be draining the battery, it's not just the G1000 that will be running but the electronic gyros, beacon, etc. If the airplane hasn't been recently flown and it's cold outside 15 minutes may be long enough that you can't crank your engine.
There's also problems with running the G1000 while starting the engine:
- It's not good for the electronics: you get all sorts of electronic noise and surges when running the starter. I don't think it's that much of a problem with modern electronics, but the G1000 is a painfully expensive piece of equipment and you want to preserve it in any way you possibly can!
- You get a power sag: the starter drains a lot of power, there may not be enough to keep the G1000 going and it may restart. It it restarts you lose all that configuration you did! You could mitigate that by saving your flight plan to an SD card, shutting down the avionics for start, then re-loading it
The last issue is that 15 minutes is time you could be flying, or someone else could be flying. It's also time you might be paying for depending on the school or group you rent from (if you rent)
There's no reason to do this anyway to be honest, you can use SkyVector, Foreflight, SkyDemon, Garmin Pilot and other flight planning software to plan your flight on a tablet or other computer, load onto an SD card, then load it on to the G1000. Some of the software costs money but it's worth every penny as it gives you the opportunity to take your time planning flights rather than rushing to get waypoints in. The interfaces are also much nicer to use than the G1000.