The decision about whether to use a front plate is made on a case-by-case basis. If you’re doing a national TV spot, you probably wouldn’t use one since, as you point out, it would look strange to some viewers. If, on the other hand, you’re shooting a regional TV spot, you would follow the local custom.
In a print piece, unlike in broadcast, you can only show a portion of the vehicle. If you’re showing the back end, you’d use a plate. If you’re showing the front end, you usually do not place a plate UNLESS your ad’s concept involves delivering some message via the plate’s letters. Like if you were nose-on and tight with a Pontiac and the front plate said “ZoomZoom,” or whatever.