There is no real consisten rule to bass compression.

Bass has a lot of energy,particularly at high volumes where its behaviour changes radically.
I deally you want to control that energy to a certain extent to stop any massive volume jumps.

So as RT said, you want to control the levels with compression.

The best way to judge it is to look at your output on the compression meter. You want to reduce the amount of movement on your output.

How much you crush the dynamics is all down to how much detail you want in your bass and how important the bass is in the mix. Ie for dubstep bass is the lead instrument, so you want to retain some dynamic and detail.

Sonalksis Stereo compressor is fantastic as you have the mode selector which allows opto and electro operation and has some lovely controls such as the transient squasher.

Rough Rider is too harsh and spikey for bass in my opinion.

If you are looking for free plugs, try Endorphin.