Depends on the rifle. My rem 700 shoots just fine with a clean bore and as suchI hunt with it that way. My knight is better fouled and hunt with it fouled. My Savage 10IIdoesn't matter so I start out clean and leave it until after season (shooting smokeless in the 10II). I keep notes how my guns shoot clean(if fouled is better) just in case i have to start out with it clean. In most cases 1-2 POI shift is all I have seen with a ML, IMO not a lot to worry about given the size of the vital target on deer.

FWIW sabotloader I do the same thing with all my rifles in regards to a thin layer of oil then dry patched to remove any excess before loading a MLor using a CF. Works for me as well and I believe some protection is required for our enviroment.

Forget the "they" is my advise, you must find out what your rifle prefers and go with that instead.