I think it all depends on where you are flying, the radar positions I work are generally all one way sectors so I never give a thought to issueing a correct directional level, I just give the highest or lowest available, but then most of the traffic I work is either climbing out or descending into airfields. Also, on the Dover sector at Swanwick, Maastrict accept a/c at all levels between FL250 and FL290 24/7, as long as those issued westbound levels are requesting a higher cruise FL than the one planned for transfer. Likewise on the Hurn sectors, to aid separation just south of the boundary in Brest's airspace, those exiting on a westbound track are allocated eastbound levels and the eastbound tracks coming into our airspace come at westbound levels. I think its something to do with safety
