I was always lead to believe that DC SP(+)? polaity gave the best 'penetration'
We don't use 'straight' and 'reverse' over here in the UK, only + and -, so it can get a bit confusing!!
However, as long as a test piece is acceptable to whatever 'code', I don't see a problem!
I have found that it is easier to weld structual steel ie 'I' beams with AC because of the arc-blow problems with DC! I think this gives a better weld overall than one done with DC (less spatter etc) But that is my thoughts only!