Going with the rules as written, QuickDraw does not allow you to quickly sheath a weapon.
But as a DM, one of the things I can honestly say is that keeping track of what weapons have been drawn and dropped is an aspect of the game that is generally glossed over. I have had players use a ranged weapon, switch to melee, and then switch back to their ranged weapon very often, and usually in situations where the AoO is not an issue. And even in situations where it possibly should have been an issue, this is easy to forget. Players will mention that they are changing weapons, but they usually not specify of they are sheathing or dropping it.
Quick Draw is usually not taken as a feat except by builds that depend on thrown weapons. If you want to make the feat more attractive by adding the sheathing as a house rule, it probably will not break your game.
Besides, when was the last time someone took an AoO for sheathing (or even drawing) a weapon in one of your games?
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