Yes, it wasn't as cut and dry like some other shows at the time. Like all Fords or all Plymouths, etc. There was a blonde young lady in the Timmy era that always had a 57 Bel Air convertible. I think it is in the hot rod episode where Lassie saves a family of, let's see...coons? The episode with the traveling rodeo guy with his horse had a 57 or 58 Mercury Commuter to tow his trailer. And in the episode "Desperate Search," the blinded boy's family drives a 1956 Plymouth Suburban. Lastly, that white '60 Ford Wagon was used in several Timmy-era episodes, including the car belonging to Joey's father (of the blinded boy) in "Desperate Search," and, in the next episode in the series, as the gas company surveyers in the episode to save Mr. Cully's barn, "Home Within A Home."

I'm sure you know of this site, if it still exists.
http://www.lassieweb.org/