The usefulness of pivotting is argued. Some people will tell you it's not worth doing, others will tell you it's good to know if you can do everything else, and others will tell you it's important. I highly recommend you learn to pivot, personally, especially for Marth. If you learn to pivot on command, you'll wonder how to ever lived without it.
Is that Falco comming down on your head with a dair again? Dashdance out and pivot into a tippered fsmash. He'll think twice next time he's above you.
Also, the alternative Eggz mentioned is worth trying out. It's a technique that some people find much easier then pivotting, while others find it absurdly hard to do. I personaly can't use Eggz's pivot, but I can do regular pivots just fine. Many people on this forum will tell you the opposite about themselves though.
EDIT: Don't get discouraged with all this "one frame window" talk. Pivotting, like waveshining (and just about every other technique in Smash) is all about practice. It sounds really hard to do at first, but it's all in the timing. Once you know the timing for it, it's a cakewalk to use. The great thing also is that pivotting follows Marth's natural rythim almost perfectly, so you kind of "flow" into them in games. It's hard to describe >_>