I started collecting SBs in the past year, and first and foremost, I have to enjoy the movie. I will open all of the SBs I buy to watch. I understand the "keeping them sealed" mentality as a collector, as I have done this with collecting action figures in the past. But as I get older, and finances become more important due to real life (family, house, etc.), I've moved into the realm of not spending money on collecting items just to pack away in a box or on a shelf. Although, there some exceptions. But with SBs anyway, I would open just to look at the inside artwork....
Second would be price - over $35 starts to put stress on the wallet. Honestly, I started collecting SBs because I got tired of losing money every time I bought a standard release. At least SBs have resell value.
Third is artwork. Generally, if i like the film, I'll usually like the artwork. I don't think I'd buy a SB movie that I'm not interested in just for the artwork though.
And I'm not for buying multiple copies or various releases of the same film, unless I'm a fanatic for the film (like Sucker Punch). They would have to be on my top 10 list of films for me to do this.
But I guess like any collection, everyone is going to have their own set of "rules" as they collect....