Can a CPA firm prepare tax returns for which fees are based upon client refund if the firm also provides attestation services to the client?

Attestation services for what exactly? i.e. in what way is that related to the taxes?

I will say I'm not sure I much trust someone who bases tax fees on a refund. Instead of being paid for how much work they're actually doing, they have an incentive to find deductions for anything and everything -- that may or may not be legitimate. While they should do everything they can to find legitimate (even if it's a little borderline), fees being based on that makes me a bit nervous. Every accountant I know of around here charges either by the hour or by the form.