I agree that you don't want your saw horses to high when doing mortice work. Especially when using a classic boring machine it's far more comfortable and safer to have your feet reach the ground. I have the sore ASSets and broken hand to support this claim.
Another thing to look at is the height of your timber cart, if it's not well matched to your horses you'll end up with one end of the timber uncomfortably low (or heavy) for lifting when the far end is on the horse.
I'm thinking of creating a somewhat longer pair of horses with a stepped up top to allow easy raising and lowering of the timber to and from layout height.
