I don't care about genre, I don't care about time period.
I collect according to the following guidelines: Movies I love (Alien, for instance) and themes (actors, directors, Academy Awards nominated flicks, etc.) I also try to get what I can snatch for little money. If I see it, I will get it, even if it doesn't fit any particular trend in my collecting. Just because it was available and I couldn't let it pass me.
I don't particularly care about Raiders per se (although I loved the movie), but I do care about Spielberg and I intend to own every single piece of material on him (posters and lobby cards, that is) as long as I don't already own the design. If I do buy the same design (and I have, it's not because I was looking for it, but more because it was available for nothing. With 870 posters and LC sets on Spielberg, I'm getting close. Same goes with Bardot, Eastwood, Hitchcock, Kubrick and many others I'm a fan of. Because I'm a completist, I will attempt to get it all. One day when I realize I own a lot of Cardinale, or a lot of Lollobrigida, I will start collecting those. When you add it all up, it means a lot of posters.
Lots of my post 90 gear is connected to Academy Awards, movies I liked, actors and freebees. If I love a movie, like The Artist, I will get every single design I can find on it.
T