All the above is good stuff about settings etc, but as I understand it, you asked basically whether to set the comp so it`s on all the time, or only works if you tan it.
As a bassist, I like a little bit of compression all the time, so that means a lower threshold and a slightly lower ratio.
I like the sound of a compressed bass.
As a sound engineer I would use compression to prevent dynamic spiking and to even out an inconsistent signal, so I`d be looking to let most of the signal through without compression with a higher threshold, but a higher ratio to pull it down a fair bit when needed.
The slower attack setting mentioned above for pick use etc are all relevant.
For fingers I would use a fairly fast attack.
As an engineer I tend to think that if I can hear compression, I`ve used too much.
As a bassist I like to hear it as part of the sound.
So it`s up to you and what you decide you like but bear in mind it`s really easy to choke down all your dynamics, with clumsy compression.
MM