The surface agitation increases gas exchange, so if you are adding CO2 you don't want much splashing, since it off-gasses the CO2 too fast. If you're not adding CO2, it won't matter as much.
I keep my tank filled pretty full (above the black trim on the top), so my HOB filter won't make loud waterfall noises. Plus, I think it looks better that way.
If you're not adding CO2, then surface agitation could actually be good since the increased gas exchange at the surface will actually increase the amount of CO2 in the water slightly, as well as ensure that the O2 levels will be sufficient with the lower plant growth/O2 production.