Are you making a new bow? or reworking an old one?
Longbow, recurve, laminated?
What is the difference in tiller measurements between the top and bottom limbs at the fadeouts?
A possible negative of 'negative tiller' is that when the bow is drawn, the weaker lower limb allows the nocking point, and nock of the arrow, to raise in relation to the arrow shelf. If it's bad enough, it could cause porpoising that is difficult or impossible to remove by adjusting nock height.
If it's not bad and it shoots an arrow well, it may not be worth worrying about. If it is substantial and you want to correct it, you may be able to remove some material from the top limb to weaken it, and/or shorten the bottom limb to strengthen it and get positive tiller back in the top limb.
Are your limbs currently of equal length or is the bottom limb already shorter?