I do NOT agree that an overfill WILL extend the useful life of a bag and if you are wise enough to buy a REALLY GOOD down bag to start, it WILL, given proper care, last all of your life.
I have owned/used topend bags from the original Marmot Mountain Works, Fairydown of NZ, Feathered Friends,Caravan
of Sweden, Integral Designs (Canadian-not Chinese), Pioneer of Vancouver, Valandre and Western Mountaineering and used others over five decades. I used my custom Marmot bag intensely from the beginning of 1978 to Dec.1999, when it was stolen and it was STILL as lofty as when I had it made.
I did wear out the fine Pioneer bag, my first good bag, bought summer of '68, but, it was still useable when I replaced it in 1978. It had lost 20-30% of loft and a seam split, but, this bag was used continually for 150-180 nights per year in the bush, wilderness working-recreating and every night as a quilt when in college.
So, I doubt that you have anything to worry about, I always store my bags, down and synthetic, in very light cotton bags on ventilated shelves and have never had ANY issues with this. I don't hang bags as the storage bags keep dust off them and are easier to find space for in our humble home.
JMHO, do as you think best, but, a WM bag will last longer than you will, bet on it.