A good knife is a MUST for a whittler. Heck, it's a MUST for a carver too. But since the whittler uses only a knife I think it's a tad more important. There's another thing that is important. Find a good knife and stick with it. Learn to maintain the blade. Learn to keep the blade sharp. The more you use the knife the more you will know its characteristics and limitations. You'll know just how tight of a stop cut curve you can make without breaking the knife tip. You'll know how often too strop the blade. When making a cut, you'll know exactly where the knife blade and tip is in the wood. When slicing up or down to a stop cut you'll make a much cleaner cut when you know where the blade and tip is when it's in the wood.
If this is all true, and I think it is; why do I keep on buying new knives?