Usually one uses the low cut when there is wind noise and/or close miking (speech) to avoid rumble and pops.
You should follow Zim's advice.
I know this is an old thread, but I have to step in and say do NOT follow Chris or Zim's advice on the use of low-cut filter. I'm no audio expert, but in my attempt to produce better audio for my recent shoot, I bought a new $300 microphone and followed the above advice only to find out that my audio was tinny and echoy as a result. All the deep, rich sounding audio that I could have gotten with the filter off I didn't get. I must admit that it's partially my fault. I decided to test the difference between having the filter on and off, but didn't realize that both my mic AND my camera have low-cut filter switches. I flipped the mic switch on and off and got the same audio so I thought I was good to go.
Once I realized this, but only after wasting a 6 day shoot and 10 tapes worth of footage, I tried the test again with the filter truly off. Suddenly, I am getting rich, deep, excellent audio as opposed to the crap I got with the low-cut filter on. Now, I'm sure there are times and places where the filter would be beneficial, but I can tell you that if you're shooting people talking inside a building or something similar, do NOT turn that little filter on. You're audio will suffer greatly.
If you don't believe me then just e-mail me and I can provide an audio file to show you the difference. Or just give it a try yourself. The difference is pretty dramatic.