Well, the way I was taught to use the radar was to takeoff with 5 degrees uptilt. Gradually decrease this so that the radar is level at 20,000ft. Still decreasing so that its at basically 2-3 degrees downtilt in the cruise.
The reason you need to have downtilt during cruise is that the radar only reflects off water, not ice crystals. And the top of Thunderstorms are usually made up of ice crystals. So if you were to go along with it level you could fly into something. Another rule of thumb, with 2-2.5 degrees downtilt if a target dissapears inside 80nm you will fly over it, if its still there inside 80nm you will fly into it.
The reason for the uptilt during initial climb is twofold. One the aircraft path is going up, so you want to see where you are going. And secondly it eliminates ground returns which would otherwise obscure genuine targets.