Surely, if a sling is allowed, then use one if you think it helps your shooting. Nobody is compelled to use one if they think they don't need it.
Me, I use a sling for standers but not for kneelers. Let me add to that; I use a sling for standers when I'm shooting FT. All my other standing shooting is done without a sling. The difference is due to the weight of the FT rig, plus the fact that I shoot FT on 35x mag.