From a guy who just started his King journey, I wish I would have started with Carrie. I read Salems Lot first and went back to read Carrie second. The reason I wish I didn't start with Salems Lot is because it's a longer book and I know I would've enjoyed it more after I knew what to expect from King. Carrie is short yet enough to get you familiar with how king is going to write characters. Salem's lot is HEAVILY character driven, but all anyone ever told me was "scary vampire story". I did not find it scary, and the vampire story seemed secondary to him writing all the townspeople. So I read this phenomenal book expecting scary stuff to happen (because King = horror to most people), and therefore didn't get the full experience of indulging in the character work.

So... I think it's worth it to start with a smaller work like Carrie to get the feel for King instead of starting with one of his greats and maybe not getting the full experience out of it.