I think it is really a matter of preference. While learning you are going to make a lot of mistakes anyway. No matter what order you put things down you should take the time after laying a new tone and compare it to what you already have. Ask yourself if the tones you have placed are distinct and if they represent what you are seeing. Try to reason out what is wrong with the tones you placed then start adjusting them. Don't be afraid to erase a whole tone and redo it until you you get them to a place you like. Stan even mentions there is no point in an art piece you can't correct things anymore.