Certainly wouldn't hurt to get a trimmer. My favorite is the Lee setup. You purchase the pilot and shellholder as a set (around $6.00). You'll need a pilot/shellholder set for each caliber that you want to trim. The cutter and lock stud are sold as a set and work for nearly all calibers. Get the cutter with the wooden ball if you can find it! If not, you can epoxy some sort of grip onto the cutter, a block of wood will do.
You just screw the shellholder onto the lock stud, then tighten the case into the shellholder. You screw the trim pilot onto the cutter. Use a cordless drill to spin the case against the trimmer/pilot until it stops cutting. Deburr the mouth of the case and your done!
The pilot/shellholder sets come in the caliber that you need to trim (7x30, 30-30, 45-70, etc). Don't forget the deburring tool. The whole setup will cost you $20.00 or so!