My personal, strong, preference is always to take the refund, even if the client then takes the funds and sends them right back in an estimated payment. I like to keep the years separate. That way, any unforeseen circumstance (the IRS sends a cp2000 letter about a form you never got, for instance) does not instantaneously mess up TWO years' worth of returns. The first by the mis-match or other problem, the second by the funds you had counted on carrying forward NOT so doing - so now the rest of the year's estimated payments are mucked up.
My business partner is learning this the hard way, with an elderly client who failed to give us some papers for 2017. The IRS took the carry-forward amount to apply to the prior year, and now this elderly lady is all confused about what *he* did wrong, that the rest of her estimated payments had to go up so much.