bit like sayin, when flattening off, do you have to get the lines out...
id say the better you get it, the better the finish..
i tend to hold it flatish to start, specialy if the walls a mile out (old houses can be all over the place) then slap it straight back on with the darby or featheredge..
i let it suck in a little bit before i go at 90, but thats freehanding...
if you got beads or screeds to work off just go straight at 90..
prolly not ideal but thats the way ive done it for yonks..