I have them on just bright enough to make the instruments fully legible without confusion in the dark. Same with the CRT's if I'm operating an A/C so equipped.
This is a fairly subjective topic, as everybody has their own personal preference. In the Dash, I prefer to have the Storm Lights on very low so as to make my depth of field perception to the flight instruments easier (and to read the TOLD card). Some prefer to have them off.
It's a matter of preference, so do what feels right for you.
