You should identify the predominant grain pattern like Steve Neul drew, not only with fir but with every species and every cut, router, planer, jointer, shaper. The only time it's not necessary is when using a saw. In those rare instances when you need to do a climb cut, you also have to work with the grain direction. It's usually the grain direction that determines how you have to make a cut. Go against it and the probability of tear out is almost guaranteed. End grain is different. You may get tear out on the exit when you rout across it in the normal way, against the rotation of the bit. That is often removed if the cut also goes down the edge afterwards. Otherwise, you use a backer piece. A climb cut on end grain can still give you tear out and/or burning.