Regarding the step-lap versus simple birdsmouth question:
the step-lap is meant to resist rafter thrust; if there's no thrust, no need to use it and a simple birdsmouth will suffice. I think you would rarely find step-laps in buildings with purlin plates, for example, traditionally. It could be argued, though, that with shrinkage there could be the potential of thrust at the rafter foot, however small. So a step-lap would be a preferable joint at the wall plate.