I gotta disagree a little bit here. Though effective rates are the "bottom-line", the higher the nominal rate the higher the "tax avoidance" costs are to the corporations and costs to our government in loss "revenue". I used the term revenue for understanding but not because I believe in such a concept at the federal level.
As far as the right level, I think we would need to agree on what we're trying to accomplish by taxing corporations. Some believe in the "revenue" theory, so taxes are used to pay our bills, while some like me believe taxes regulate the growth of the economy.