When you are learning new songs, do you first practice songs you know to the point when you can play it perfectly, and then learn new stuff? Or do you learn a bunch of songs even if you didn't completly mastered the old ones?
I think it's a waste of time to try to learn a song "perfectly," as such a thing couldn't really exist. Just play it and practice it until you're satisfied with where you are. If you think you can improve, then do it.

There's the typical artsy-fartsy answer.

As for myself, I am guilty of starting songs and not finishing them. I'd say it's my greatest weakness as a musician. To get ahead you'll need concentration and determination to complete a goal--don't put if off.

Now, if only I could practice what I preach..
Quote by Trowzaa
I wish I was American.

~ A Rolling Potato Gathers No Moss ~
I learn what I want to learn. Whole songs or just a riff or two/solo. Depends. Problem is though when you learn full songs from start to finish and then don't play them for a few years you have to relearn how to play them again, but it usually doesn't take much time to do so.
When I set down to learn a song I set out to learn the complete song. I may not get it to perfection, but I try to learn all the riffs. One thing that may prevent me for learning the song completely is that I may need to take time to work on a technique. I might fly through learning most of the song easily but a small part could trip me up. If this happens I take time to work on this technique and find songs that have the same technique in it.