I think that each of these have their merits:
BONE FOLDER: No matter how you fold, this will break the fibers more even when pressing on a manually folded card.
SCORING: A personal cutter with a scoring blade is the minimum you must have. Making boxes & some cards require pre-scoring. A Scor-Pal is nice, but try the scoring blade first. The use a bone folder too & you're pressed real good!
MANUAL: You always know where your hands are! I honestly prefer to fold manually because cardstock isn't always a true size. I match 2 corners & pinch the fold. Then I match the other 2 corners & pinch the fold. Then I press between them & use a bone folder when I remember. The only time that you get a warbled fold is when you go against the grain of the paper & you don't find out about that until you've done it. Press it out with a bone folder will help.
I use a bone folder in my latest video on an easy way to make a Double Fold Pocket Card.
Check it out here.