We currently allow shooting and movement over but not under because of angle issues. Another aspect of the debate is a prone model, they may lose cover altogether, even if they have crawled under the car, because they are not in contact with it. One alternative to round bases is a semi transparent overhead silhouette that looks like a shadow. Make it clearer when a model has cover, but restricts conventional bases to playable trooper models only.
From a practical standpoint I play terrain vehicles as solid boxes as far as the gap between wheels the ground and top chassis is concerned, the same goes with any building elements that share the same issue.
It depends on your community. I find fun social groups will all agree to not shoot beneath it. But if you want to have something concrete to avoid arguments I'd put an I shaped piece on the bottom, blocking any line of sight beneath it. Make sure it connects to each tire.
