You always want to start from one corner and finish at another corner. Starting in the middle of a wall can lead to a lap mark showing up on the wall when the paint is dry. While rolling your walls it is extremely important that you keep loading your roller with paint and keep the walls saturated.
During my years of teaching new apprentices to paint, the most common mistake is not loading the roller enough. One of the effects of not loading the roller enough is what we call dry napping. What happens is your roller is unloaded of paint and begins to make a louder hissing sound instead of a sloppy wet paint sound.
This sound indicator is notifying you that your roller is no longer unloading its paint but picking paint up off the wall you have already laid out. The roller wants to stay wet with paint so it will remove paint if it becomes too dry. You can read my entire article with the link below.