I think the answer to this question will be depend on: how much are you paying for the gun, what is your risk tolerance, and how much of a project are you willing and able to take on if the gun has "a surprise inside"?
Knowing what I don't know, I have had all my used gun purchases checked by a smith. Twice this found issues that I possibly would have missed - in both cases the seller responded to the gunsmith's report by agreeing to pay for substantial repairs.
I started out with a local smith who seemed knowledgeable and was well regarded (locally at least), however he failed to notice that a gun I bought had been substantially overbored (by 0.021") and then jug-choked. This came to light when I tried to sell the gun and the buyer had a more diligent gunsmith. Since then, I have NECG do the inspections. It is not as convenient (2+ hr drive away) but I am happy to trade the convenience for more confidence.
