Hey Lee, this isn't in Lisa's forum so I'll answer.

One finger per fret is very good fundamental discipline to learn. It builds strength and agility, stretches those hands out, gets the muscle memory going.

Once you develop that basic skill you'll start breaking that rule and playing what best fits the piece of music and is efficient (but also what happens to feel good for you!). I find that when I get comfortable with a piece of music and start "rocking out" I will get very loose with all of that and even play whole phrases with one finger.

To sum it up, one finger per fret is an important "training wheel" you should stick to until you feel ready to ride wheelies around the block. :)

-Carl.