is there a movie you watch once a year, every year, perhaps on some anniversary?
Generally the original Star Wars trilogy and the LotR trilogy. And, every Fifth of November, V for Vendetta.
do you switch off the lights and the phones and unplug the door bells?
Lights? Yes. Everything else? I wish.
if there are sequels, do you try and watch all of them together?
Only if it's Star Wars, LotR or Indiana Jones. As has been said, sequel decay tends to set in with just about everything else.
can you just watch #2 or #5 in a series, or do you have to watch all in order?
I skip the first Star Trek film on principle. 
can you watch over other friend's houses, or do you have to have it perfect at home?
I like talking about the movie as it plays, so anywhere I can do that is fine by me.
do the blu-ray snobs refuse to watch DVDs now?
I'm a proud DVD snob, thank you very much. 
with certain movies, can you just watch a scene or do you have to watch the entire movie?
I can generally just stick with the Joker scenes with The Dark Knight, but still tend to watch the whole film, as I only put it in when I'm in the mood for an overlong, draining, epic viewing experience.