I'd say you do if you make them to your specs and to fit your hand. The thing is that when someone but you fits the handle, unless you are there to grip and re-grip until it's just right it just won't be a perfect fit in my opinion.
That's just me though. I'd rather have a good knife that fits me than any knife that doesn't.
That's the reason I buy blanks and do the handles myself so they fit and balance in my hand properly.
I once upon a time bought sambar stocks for my Ruger Blackhawk and I must have fired hundreds of rounds and re-adjusted the profile many times before I was happy with the way it felt in my hand.
With knives I like the blade canted inward from the handle a little on some knives and either up or down depending on the knife and purpose.
The two knives on my avitar? (picture on left) may not look all that great to some but I gripped several antler racks in several places before I cut. It turned out they both came from the same side of the same rack. The cutter (left) has a straight handle and the skinner is a curved section. I had to add weight in both handles to get the balance right as well as other modifications but I can and have used both for several hours at a time without undue hand fatigue. I wouldn't trade or sell either one of them.
Edit: That being said If it's a knife that's not going to be used for extended periods like a hunting knife then I'm not as picky as to fit and balance.