So what you want to do is spread it out in the middle and then come back and fill it in. That will stop a thick bead of paint from building up at the top and bottom of where you are rolling at.
Now here is another scenario. Have you ever watched HGTV when they have a show involving painting? Well sometimes you may see them with the roller going every which way. Maybe you have even done this at home. It just needs to get covered up right? Well you get what you see below which is incorrect. This can lead to lines in the paint and areas being thicker than others etc...