IMHO your barrel will handle steel shot just fine. 870 barrels are well made, the more open choke though the better. A full choke maybe too tight after lots of shells, it may bluge slightly. Hope this helps.
The new steel loads have better wads than the old days. You won't hurt your barrel unless you have one of the old guns with .040 full choke constriction, and then you would still be Ok w/ limited shooting. Mod Choke would be more forgiving. If your hunting w/ it, buy Bismuth or Tungsten/matrix and don't worry about it.