Possibly. but also because BM defense radars must look at a single quadrant all the time, without interruptions. And since land-based BM defense units would generally get too expensive and too impractical to feature several powerful radars, covering all quadrants, a single radar is generally issued to said land based ABM defenses. Which then means that a single quadrant is really protected, and thus it can actually be beneficial if the missile launcher is already slanted in said direction. There is no need for the missiles to be ready to go in any which way from the start.