[racket] recursion style question

Hi guys,
Which is preferable? 

(define (foo let … (acc empty)) … (foo (rest let) ... (cons …. acc))

or 

(define (foo lst ...) (let loop ([lst lst] …[acc empty]) … (loop (rest let) … (cons …. acc)))

On the one hand we eliminate the named let construct, but on the other hand we expose the acc in the function / contract.  If the acc is not something the caller would ever need to know about, should we hide it inside the definition?

-Kevin