Check the "vertical beamwidth" specification of the radar you're planning to install. Most small Furunos have a 25 degree vertical beamwidth, and even that isn't cast in granite. (It's still useful at higher angles, especially at short ranges) I expect it's about the same angle for all smaller radars.
So if you expect to be charging along with a major up angle in the dark or fog or in heavy rain while on flat seas or still lakes, a 5-7 degree downward tilt would improve reception.
I passed on getting a tilted mount for the more aesthetically pleasing level installation.
Don