doesnt take much to flatten in anyway, just run from side to side down the wall, 2 minutes literally, doesnt have to be 100 percent perfect, just have the lines out..
you can then top it thinner and if youve mixed up fresh for it you get ages with the set....
i just find if i dont flatten it first, theres not really much point topping it cos i end up draggin em both together anyway? so one coat and let it pull a bit more before flattening it?
just depends on how f'cked i am to start with, what the final finish standard has to meet, what the background is, how much time ive got before i need to be gone and how much beer ive had the night before.. (that should have been first on the list actually ;D)