I get my favorite songs and usually branch out to the rest of the album, then to the discography if I enjoy it.
Same.
But usually I don't end up listening to the entire discography. I usually find songs that I really like by putting my library on shuffle, then falling asleep while it is playing, then waking up in the middle of a song I have never heard before, realizing that I really like it, and then getting addicted to it.
I always get like 1 or 2 songs and then I get addicted and get heaps of their albums. It gives off the wrong impression when people see my library. At one point I was really into Cradle Of Filth and I got like 6 albums and now I haven't listened to them in like 2 months so everyone assumes I'm a massive fan.
I don't set myself rules for listening to music. If the album is crap and has a few good tracks i just pick those out. If it's a well crafted masterpiece i just put on the whole thing and hit play.