Is it good or bad to completely discharge your batteries every so often ?

For lipo that is bad.

Never discharge a Lipo below 3.2 volts per cell.

If you are looking to boost the performance of your pack (particularly for stock racing), then you want to do what is called "cycling" your batteries under load, this technique is discussed in more detail here at about 7 min into the clip:

what do you mean by completely?

0 volts per cell is bad. really less than 3.2 volts per cell is pretty bad as well. i aim to never let a battery run or discharge below 3.2-3.5v per cell. for a 2S battery that's 6.4 to 7.0v for the pack. when i cycle my batteries in order to keep the internal resistance (IR) low, i discharge to 3.5v per cell.