I looked at the quality of my chips, the wear on the machine and myself and the cost of my time to readjust the shear bar and decided that replacing knives more often is the best choice. I have double sided knives for a BC2000 so they are expensive but I have found it worth it. The long run is that I always have a good set of knives, good bolts, etc. If they get worn or I suspect that an impact may have weakened them, I am apt to replace vs. take a chance of a knife coming apart.
It gets expensive but a good impact wrench and a torque wrench get me back up with minimal time, no worry about getting the dull blades into a shop etc. I did look at occasionally lightly belt sanding a dull set of knives to use on "trash" jobs where the chip quality isn't important.
I chip mostly whole trees minus the stump so sharp knives without a lot of nicks is important to me personally. I hate messing with the shear bar and repeated sharpening of a kniife will make my chips bigger and stringier (sp?) and that causes the chute to clog more.
I also use a knife sharpener to touch up the knives edge every few hours or as needed. System has worked good for me BUT I have a collection of so so knives that could be resharpened and are actually better than some of the tradeins I got from my Vermeer dealer (minus the sharp edge).