the honest truth is that there is NO safe level of nitrate.. we do water changes to keep it as low as we can, but that is all we are doing. but because it's not caustic or poisonous like ammonia and nitrite are and keeping plants alive in an aquarium is so difficult for some of us (me too.. i kill em all). we are left with doing our best through water changes.
it's kinda like smoking.. if you smoke, you KNOW it's bad for you, but your world is so filled with the smoke that you don't realize how terribly it is affecting your life. and it doesn't kill you right away, it takes it's time and slowly, and quite often painfully kills you.
this is why testing your water and doing water changes is so very important.