with a well built and balanced machine the swashplate should be PERFECTLY level. this means that you've corrected the cog issue with it being tail heavy and anything else. if you're flying your heli stable with a non level swash go back and find out what is wrong. don't use trims to correct for a bad mechanical set up.

if the swashplate is not level then you're losing some ablility, for instance, if you have the swashplate leaning forward to correct for the tail heaviness then you've lost some of that throw, this means you have more ability to pull the nose up than to push it down.

don't take short cuts, do it right.