well i have done a lot of ruling off and ruling in. but the only time i use a straight edge is to rule a screed.
for walls either internal or external. i know from the back ground what suction to expect. if it is low suction, i will rule the wall with the backedge of the featheredge, to leave the material open to let more air into it to dry quicker.
on a higher suction i would use the feather side to rule off the rule in. i have never found a use for a derby float, an I section,or speedskim on a wall.
years ago, the derby float was used when we floated ceilings.