No, it's not. You can reset it with Forscan or by using this procedure, no tools required.Supposedly, the 8-hour wait is in lieu of resetting the BMS (which a dealer would do to get the car out of their shop).
Don't perform this procedure just for fun, only when you install a new battery.
Again, if you have Forscan you can also monitor it.
BMS reset - no tools required | 2015+ S550 Mustang Forum (GT, EcoBoost, GT350, GT500, Bullitt, Mach 1) - Mustang6G.com
The 8 hour wait is just what the BMS does every time you don't use the car for... 8 hours... to fine tune it's registered State Of Charge. The BMS reset procedure zeroes the age of the battery for the charging algorithm.
It won't explode if you don't do it, it will just slightly overcharge the new battery.