I never saw the movie in theaters. I did read the novelization, as I often did back then, since my parents wouldn't take me to movies as often as I wanted to go (the bums). I was still living at home, at age 19, and working in a bookstore (so I got discounts on the novelizations) and didn't have a driver's lisence quite yet (noone would teach me to drive! Finally took a Sears course!).
I was a big part of CompuServe's Sci-Fi forum at the time.
There were several discussion threads always going at once about the movie BTTF.
I *distincly* remember certain things, like "during the start of the movie, could you see a duplicate Marty McFly looking on from the bushes of the mall parking lot as he watched himself disappear?" (when the film came to cable, I looked hard to figure out what people had been talking about)
Another common theme of those discussions was "what's up with the To Be Continued?...is there a sequel already in the works?"
The conclusion was that the producers WANTED to leave it open-ended for a sequel, but noone knew when it would be made. Of course, years later Speilberg and the two Bobs finally got it together and did two sequels at once. That's still a great trivia question: Who did back-to-back films in a trilogy piece long before Peter Jackson did?
Anyway, I don't know if the To Be Continued was in ALL the prints, but it had to be in some - or many - of them, judging by the CompuServe discussions at the time!