Be sure the bath wall is flat enough before tiling behind!
If the bath wall runs out, or is not at 90 deg to the end wall(s) then you
may have to chop the bath into the wall a bit to get a decent job. In this
case definitely tile down onto bath edge.
Recommend set out tiling accurately leaving nearly a full tile above bath
edge line.
Tile completely except row above bath then install bath and fit final row of
tiles cutting bottom edge as necessary.
This avoids having to work above bath and reduces possiblility of dropping
tile and damaging bath.
Use top grade silicon no cheap swag.

Phil