My considered and irrefutable opinion. It depends. In general, I like a knife, or firearm for that matter, that tells a story. If the items shows honest wear and even hard use, I am inclined to clean it lightly, remove rust, oil it up. Then either use it or add it to a display. If a valued (not the same as valuable) knife blade has been subject to someone's power grinder, I would consider having a professional work on it.
I had this choice recently. I picked up a Buck 124 with sheath for $50 at a gun show. It was splattered with what looked like JB Weld that had been mixed on the back side of the sheath. The guard and butt were scratched and dinged, as were the wood handle slabs. My first thought was to send it to Buck for the Spa treatment. I've send in a badly damaged 102 Woodsman and a 112 with a broken lock, and been very happy with the results. However, I went to work on it myself. Polished out the grind marks where someone had tried to sharpen the top of the end of the blade (swedge?). Polished the aluminum and the handle. Polished the blade to remove some scratches. The JB Weld or whatever come off the knife and the sheath with only minor remaining damage to the sheath. I dyed the sheath with black leather dye and put on some leather conditioner. Sharpened the blade. I'm very happy with the results.
I also know that collector value, especially in firearms, is reduced if it's refinished.