Trimming wasn't mentioned in the manual because back in the old days, the film leaders were cut longer and thus fit perfectly.
I'd say trimming is recommended. Sure you can do it in other ways without trimming, such asusing the card or another trick like sticking your fingers through the lens mount to adjust the film, but I'm not too crazy about sticking things in the camera that don't belong. So trim the leader and enjoy foolproof, easy and safe loading every time.
Nikon S2, S3, F, F2, F3, FM2, FA, N90S, D80, D7000, D750, Sony a6000, Canon IIf, Leica CL, Tower type 3, Zorki 4, Vito B, Perkeo II, Rollei 35....