I have installed three different radars on our boat. First was a small, 2kw dome, then upgraded to a larger, 4kw dome, and now a Lowrance 3G.
In all cases I found lots of good info in the installation instructions that came with the radar. Guidance on beam width, beam height, clearing obstructions or the hardtop, etc.
In general, most folks lean them forward so the beam is less impeded forward by the leading edge of the hardtop, and the angle can be "down" 4*, or 6*, or whatever, so the radar is closer to level when the boat is riding bow up. But I agree with the comment about moving slow when you need to see in close to the bow.
Most radars have a vertical beam width (height) of 25*, so you really don't need to raise the radar to see forward as long as the radar is close to the front of the hardtop. You may get an improvement in contact returns on the beams of the boat by raising the radar, if the hardtop is wide. Again, very clear charts in the installation manual for the 3G radar.
Some pictures.