Yep, take a run towards the grid and bunny hop to get the bike airborne, I always do this as fast as I can, I approach it like a sprint... I have never come off on a cattle grid yet even in the rain.
Im not certain of the distances Im clearing as I have never measured them but equally I do not stop and check if I am approaching a sewer grid or cattle grid (and would not know the difference anyway).
I just have a go at it for no other reason than its fun.