Each kiln is different, but if your kiln fires nicely, you can load it just about any way you want. The only thing I would avoid is putting very short shelves at the very bottom and getting too close to the walls. But I've never had a problem with appendages hanging out over the bag walls as far as kiln performance is concerned. I've had the good fortune to fire dependable gas kilns over the years, and have always stacked them as tight as possible, with less than 1/8" between pieces. If it's designed properly, there will be good air flow through the kiln even if the stack is very tight. You do not have to be all that concerned about flame path like you do with a wood burning kiln.
In a downdraft kiln, the bag wall is designed to force the flame upward before it is pulled down and out the flue. The height and tightness of the bag wall will depend on the kiln. For a new homemade kiln it will take a couple of firings to figure it out. If the top runs cold, the bag wall is built higher.