Using a sling while standing can steady your shooting a bit, but not enough that a 'cuff sling' is warranted. By 'cuff sling' I mean the kind that you tighten around your arm- I've use both a G.I. M1 canvas sling and a leather 1907 sling for high power shooting (prone, sitting), but don't see using them as a cuff sling for hunting. When you tighten it down around your arm, you are often times cutting off the circulation to your arm and you usually need a heavy leather glove to keep the sling from tearing up your supporting hand- not such a good thing if you are sitting in a treestand
If you want to try using a sling to support your rifle while standing, hold the rifle in your right hand around the wrist of the wrifle stock and let the sling (attached to both ends of the rifle) dangle freely. put your left hand into the gap between the sling and rifle, then around the bottom of the sling and around until you can grip the forearm of the rifle (Its hard to explain, but if you lead the insertion of your arm with your elbow, it will be much easier). The sling should have a bit of tension but not enough to pull the butstock away from your shoulder. Try this at the range to see if this works for you- there is some controversy as to whether or not a sling helps with standing shooting, I find that it helps.