You create a slippery slope with exceptions.
I don't think so. Not as long as they are 100% non discriminatory and all citizens can take advantage of them. Slippery slopes develop and become a problem when you start saying that this group gets this but not that one, etc. etc. etc.
As long as Congress can't use the tax code to 'buy or manipulate votes' from special interests or targeted constituencies or certain states but not all, etc., it can safely use the tax code to promote desired behaviors without corrupting either congress or recipients.