I never play 2 STs. Some of those lineups refers to an AMC as the '1' and not a ST meaning I'm playing sans ST. The reason being that an AMC helps out with defensive tasks, which a ST doesn't. So it depends on opposition strength and midfield.
The amount of players in the back line depends on the opponent. If the opponent is playing ST AML AMR I play DL DC DR, if STx2 then I play classic DL DC DC DR with a DMC etc. It really depends on the opposition and whether I want to overcrowd the center or flanks.