I would advise ticking it - the difference being that your URLS will then look like

https://www.ubbcentral.com/forums/ubbthreads.php/ubb/showflat/Number/173872/Main/38820/#Post173872

as opposed to containing other characters - the / replaces them, and assists spiders in crawling your site.

However not all servers can display these - so if you get issues, then deselect.