I always thought it was the law to tell them right away.
Like most things involving firearms regulations, it varies by the state.
Apparently not if he asked me as a "courtesy", the implication being it was not required. Plus, a retired state highway patrol trooper, a hometown city police officer and the county sheriff told me I didn't have to tell him anything. At least in Idaho that seems to be the case.
That depends on the state, and is something to be aware of if you are traveling through multiple states. (Apart of course from the other issues related to multi-state CCW, like license reciprocity.)
In some states there is even a contradiction between state law and federal law regarding travel with a firearm stored in the car. Read up on the law before travel, and seek competent legal counsel for the complicated cases. Oh, and avoid New York, New Jersey, Connecticut...
that is not correct. many choose to as a courtesy.