If the timing is retarded by a small amount you will notice a gain in torque the 'low' rpm range, advancing it will shift the power to the upper rpm range.
That being said, a tooth off on a timing belt is a fair amount, & would agree either direction will run poorly.
There is no such thing as a safe amount, all depends on how close pistons and valves get, generally you can dial things in with in a couple degrees with out trouble, although any sane person would make the adjustment and rotate by hand if in doubt.