Straight wall cases as a rule don't lengthen, although it can be nice to clean up any small cracks that might appear in the case mouth. Bottle neck cases lengthen every time they are fired and resized. Trimming keeps the round easy to feed, and prevents excessive pressure caused by the neck swaging into the bullet shank when the case lengthens enough to enter the lead of the barrel.
So yes, a trimmer is an essential piece of equipment for the handloader. There are 3 solutions you can consider. The Lee Trimmer consists of a base that can be chucked in a cordless drill, a cutter, a cartridge stem, and shell holder. It works, and can be fairly fast, although sometimes it is a pain to get the case to spin concentrically. The stem screws into the cutter and provides the correct cutting depth for the specific cartridge. After cutting the case must be chamferred inside and out and the RCBS or Lyman chamfer tools seem to be the best.
Next up is the lathe type rotary cutters. The very best are probably Wilson, followed by Forster and RCBS. These also require chamfering after trimming.
The last type are the powered case trimmers typified by Giraud and Gracie. Dillon makes one and the big die makers seem to offer some sort of powered trimmers now, but I've never looked at them. These machines tend to be expensive, but trim cases very uniformly and very quickly. In the case of the Giraud and the Gracie, there is no requirement to chamfer as the cutter performs both operations. I don't know if this is true of the other powered trimmers.
Edited to add:
I've found both H-4350 and H-4831 to be very good in the .300 Winchester. I still use Unique in the .45 Auto, but it leaves a sticky residue and as a result has become less popular with many handloaders.