If you're talking about the aux deflector, there's a reason it's called "auxiliary."
I'm thinking it only lights up at high warp or when the saucer separates. Your mileage may vary, however.
I'm with you 100% on this point. Lighting things up just to have "neon lights" is silly.
If it's in operation, light it up. If not, leave it dark. (And, if you can... another preference from my personal pref category... I'd love it if you did the TMP-type glow, where it "warms up" from grey to dull-orange to yellow, to white, to blue when at max power. Your call obviously, just casting my 2cent vote!)