People do it both ways but as a general rule you would be better off with some type of plan . In a lot of cases it's used as a guide line but it still gives you a good idea of what you are aiming for. Even with a plan things do change or you find that it either doesn't fit or look correct and this is where you change it to fit/correct the problem but over all a plan is the best way to go.
Just my opinion and yes I have done it both ways.
Jim