Restrictions are usually "No lights higher than the top of the dashboard"....but height usually helps. "As far back as possible" important too, as "bump-drafting" (or its evil twin "incidental contact") with a $500+ set of lights is sub-obtimal.

Consider "easily removable", too. No point crashing a $500+ set of lights at 1pm. But people do it anyway. We always had a quick disconnect light bar...we saved a lot of headaches and $$$ with that bar.