If anyone, other than you are likely to use the computer you are visiting PPRuNe from then you should most definitely log off. If you are the sole user of your computer then there is no need.
Worst case scenario is someone who logs on from a public access computer at an aviation related workplace and then forgets to log off. Odds are that there will be other PPRuNers at that location who are likely to use that same computer to access PPRuNe and voila! You have the recipe for someone to either out you or worse, post all sorts of things in your name which can lead to all sorts of difficulties.
You have been warned.
