There's nothing at all wrong with a negative tiller if that's what results from balancing a bow's limb strengths relative to your holds on bow and string. Balanced is as balanced does.
A bow's side profile at brace, i.e. tiller, should not be predetermined because some bows will be balanced with a positive tiller, some even, some negative. A tiller measurement should be an afterthought, or postscript... a final result of our balancing efforts during tillering. A bow that 'lifts the arrow off your hand at the shot' is not balanced and it's inflicting porpoising into the arrow's flight. My bows are balanced, the arrow flies straight away, and clears my hand just fine. Don't flip it or weaken the top limb simply because you've been taught to be biased toward even and negative tiller. You may be making a lesser bow if you do.
I say, if you don't have your tillering tree set up to adequately mimic your fulcrums on bow and string, then you should draw it, learn to feel how it acts during the draw, and take pictures from both sides to study. It should not lean forward or back any noticeable amount. Weaken a limb if it pulls its end of the handle toward you until it's balanced at full draw. Then shoot it.
By the way, the limbs don't have to be a perfect mirror image of one another, and often they absolutely should not be.
