I usually pack my sleepingbag on the seat and use the rack for bulky, wet and smelling items I don’t want to carry in the boxes. If I have to I can also place my tent back there and use the place where I normally keep the tent to carry 10L extra petrol (front of right box).
A rack makes everything easier and it doesn’t add much to the weight.
Placing heavy items in the rear of the bike is really bad for performance and it stresses your subframe a lot. A sleepingbag is about the same weight as a bottle of water, so it’s not that bad.
IMHO it’s a PITA to carry tires and I don’t see the point in doing it for long distances unless it’s more then 10kkm between the dealer/post offices.