Most IDPA and IPSC matches that use steel targets consider 10 yards to be the minimum safe distance to prevent lead splatters from coming back and hitting you. Don't shoot steel jacketed ammo like 9MM Makarov, Tokarev ammo, cheap European surplus, that sort of thing. Test the ammo with a magnet if you are not sure of the jacket material.
If a steel target does get pock marked from shooting it with something you shouldn't like a jacketed rifle bullet, then only shoot it with rifle only from then on, and at longer ranges like 50 yards. The pock mark if hit with a pistol bullet can make fragments come straight back.
I used to have some milder steel I would shoot with rifle and pistol, using the pock marked side for rifle shooting, the smooth side for pistol and shotgun.
I have a bunch of mild steel targets now that I use strictly with cast lead bullets, .30 caliber rifle up to 1700 feet per second.
Check out this Ebay seller, "60RPG49":
(12) Steel 6" Round 1/4" A36 Pl Shooting Hunting Target - eBay (item 120667370667 end time Feb-02-11 16:41:33 PST)
He sold me 25 pieces of 4" round steel to use with my youth shooting team, 4" is close to the size of the aiming black of a 50 yard Smallbore Rifle Target. I'll use these for .22 rifle games like bicycle biathlon, team speed shooting events, etc. Kids love shooting steel...