I'm really careful the first time I ski a trail with big whalebacks. The back side is often much more damp/icy than the front side, just due to the draining of the water. I do this even if the front side is very powdery. As Brownski said on the cover a couple of weeks ago, treat each gun as its own microclimate. Just because the first and second are good, it doesn't mean that the third will be too.
Like skiing moguls, as you reach the apex of the bump, your skis are likely to have very little of the surface on the snow (relative to a regular turn). Thus, your skis can pivot much more easily. This is good to keep in mind because it's easier to make a turn. In other words, with so much of your skis having air underneath, don't over-turn.
I rarely go right over the top of a whaleback, instead opting to ski the side away from the hydrant. This is because the mound is not uniform in its curvature. The portion toward the nozzle is much more pronounced (and likely to be damp), especially for a large mound of snow made by a Ratnik (ground gun).