| I think it's all to do with your hands on your bass, and how your hands move. Everyone is different.
My hands are kind of medium sized, so I quite often use my pinkie when playing in those lower positions, so I've got 4 fingers over a 3 fret range. This works for me with the kind of lines I play.
I try to work on my fingerings so they are consistent though, fingers 2 and 3 (middle,ring) sort of floating about to finger whatever fret is most comfortable.
Personally, I prefer to shift my hand a bit rather than do big stretches. I think it's something one should be able to do, rather than always being totally constrained by the 1 finger 1 fret rule. If I was playing jazzier stuff and not singing maybe I'd follow the rule more. As it is I'm doing rock/pop so those spider fingers aren't so needed.
And, as you've discovered, chord fingerings are different to line playing fingerings. It makes sense. Imagine playing the 10th type "Walk on the Wild Side" Chord with 1st and 2nd fingers - to much of a stretch, you HAVE to do it 1st and 4th finger. | 
