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It depends on you Bc and Be. (In your case, as you're using shape average, Be shouldn't apply.)
The shaper allows ingress to egress traffic at line rate and/or might dequeue at its maximum PPS rate. A burst that's "fits" within Bc (or if shape peek Bc+Be) could queue in the tx-ring.
Considering the performance capacity of an 891 and assuming you don't have LAN ingress bandwidth that oversubscribes the egress gig, I doubt there would be the need for deep tx-ring.
BTW, some of the later IOSs are supposed to minimize tx-ring when a service policy is applied. (This to minimize FIFO tx-ring queuing, before packets are placed into an interface policy [without shaper].) Are you sure tx-ring size is still 256?
