The direction of the FL according semi circular rule is not the key factor here ( We modify this all the time ad-hoc to ensure separation or expedite traffic )
There is always a danger in this thinking when not anticipating that loss of radar contact or comm failure may occur at some point. Exactly the same thinking was bringing the TU-154 and C-141 down.
Away from high dense airspaces and with the exception of the oceanic rush routes it is good practice to stick to semi circular rules over long thin traffic routes. No reason to not do so.