If you have all the controller stuff off (like input shortcuts) then you have to be very sure you let go of down first. If you accidentally only roll to db instead of back, then it probably won't work out. Starting from a down position makes it super easy to actually do down-back, forward instead of back, forward.
Interestingly enough, with input shortcuts on, you never have to get all the way to back. You can get it to do most inputs simply doing d, db, d, df, button. That will do both back forward and down forward attacks. It makes Down pokes into special pretty easy, but may give you other stuff you weren't expecting if you don't really understand what leads to what.
On my button only controller (Hitbox like thing), I can actually hold down the whole time, and do back then forward while holding down and get all my specials without ever letting go of down. It's kind of wild.