I'd say that for any monolever bike to grease them annually as the joint moves with any/all vertical movement of the final drive. Secondly, it's my understanding that the material that BMW used to make the splines is not quite as hard as what they used in the 16v splines. Don't get me wrong, they are good, but BMW made them better when they built the 16v ones.
As far as the paralever bikes go, the only spline that moves in/out when riding is the middle splines between the 2 sections of shaft. The transmission end and the final drive end are fastened with a circlip so that once you "pop" on the shaft to the spline, it doesn't move again till you pull it off to remove the shaft. Don't get me wrong, the middle spline still needs lubricated, but generally only at the service recommended interval.