This.

You *can* use a BB spacer behind your cog but this should only be done in the extremely rare situation of your cog itself being too thin for your lockring to make contact with it when everything is tightened properly. I've seen it once in my life - on a Formula hub/old Surly cog - where a 1mm spacer was needed to fill a gap and make everything work as it should.

BB spacers should not be used to adjust chainline.