I think it really depends on how your records are stored. If you file LPs like books on shelves, with spine facing out, then unquestionably it has to be top open. If there's a shelf above, then no dust will fall inside. Plus, then when you pull em out, the sleeve won't come off (I hate when other people file them with side open and the record and everything gets separated the next time you pull it out.)
BUT... 7"s, which I, like most other sane people, have stored in boxes, I file side-open. For one, if you have the boxes uncovered, then all the dust would fall right in the top of the open sleeve. Two, if you're flipping through from the top, the open sleeves get battered (I remember at the radio station, all the covers were thrashed at the top from fingers going into the sleeve). Plus, since most older 7" sleeves open from the top, having the plastic sleeve open on the side protects them better.
Funny thread.