eh...I was taught a sling is used only when the slinged elbow is supported (i.e. sitting, prone, etc), so it's not used when standing.
Also, there's a difference between the NRA Standing position, used in formal target shooting, and the offhand, unsupported position. The latter is a field position only used when a quick snap shot is needed, otherwise, a more stable position is used. So, which are you interested in?