I believe when it comes to the coax/hull mg's for a gv, the tracers won't show if they have it set to off. However, when it comes to the main guns (wirble's 20's, ost's 37 and M-16's 50's included), I ALWAYS see the tracers. Same from any manned gun. When it comes to gv's/manned guns, turning off tracers just makes it more difficult for you, since they will see them even when you don't. Least this is how it is for me (I always have tracers turned on. My aim is so horrid that I'm lucky I can hit the ground, flying into it guns blazing

).
As bustr pointed out, if you have tracers turned off, try turning them on.