You won't get ring-bulges with steel if you are not choked too tight, don't use very large shot (T or F) and don't have screw-ins. .020 has always been safe for me, but go with IC/LM if you want to be safe.

I don't like screw-ins in a hunting gun unless you just have to have a whole lot of flexibility, like "I want to hunt ruffs one day and pass-shoot ducks and geese the next."

If it was just going to be a duck gun and I wanted to shoot steel, I'd just have the chokes opened up to IC/LM.