On the rifle it came on, slacked off per regs.
If acquired separately then coiled and secured with a pipe cleaner unless too fragile, in which case hung or laid out flat (depending on how fragile it is) in a box I made for the purpose.
On the rifles. I only have a couple legit ones, to be honest. Some rifles don't have slings at all on them. Some have repros, or postwar West German slings (best "repro" in my opinion, if you want to call it that.)
I am increasingly having a hard time paying what originals bring, so I live without on a few rifles for now.