PP – my tips are:
1) clean the area that you are going to be masking VERY VERY well before applying your first coat. Sugar soap quite strong mix. It’s the poor bond between your first coat and the old wall that allows the paint to peel
2) this then lets you mask and run your finger hard along the edge where the paint colours will meet so that you get no feathering. You dont have to stick down the other side very hard at all.
3) Mask so that if the line isnt dead level the ceiling white runs down into the wall colour – the bright whit paint will hide the fact that the 90deg angle might not be the same line as the colour boundary. Having colour creep up onto the ceiling is horrible.
Oh and when removing masking tape, pull it flat to the wall (i.e. folded over so that it’s outer face is against the wall face)
Of course if youre steady as a rock you can cut in well with really good quality brushes – better than you think you can and as good as masking too.