To answer your question first we must establish what QoS is, I like to think of it as a mechanism for providing a prioritization of traffic over interfaces where there is contention.
With that in mind where does it make sense to apply QoS? To answer that you have to define where your trust boundry is, what is the limit of your control and how do you identify what traffic falls within each classification. In most Cisco VOIP deployments they push the classification out to the access layer on the switchports directly conected to end points.
Once you have classified the traffic where does it make sense to create QoS policies to carve out bandwidth to allocate to the different classifications? In short, where it counts. Trunk ports between switches do not need QoS policies because the classified traffic is already being prioritized in hardware queues. Uplinks to slower media is an ideal location for QoS because the ingress to the router could be 100M or 1G but the egress could be 1.5M. QoS would only be placed on the outside interface in the egress direction, it wouldn't make sense to put it ingress on the Serial link because the traffic has already be0en3 classified and dealt with before it got to you by your carrier.
In short configure QoS on the egress of WAN links because routers use software queues, and configure QoS classification on ingress ports of switches and let the switches handle the prioritization for local traffic.
Cisc ohas a great design guide for QoS, but just because they talk about a QoS design with 11 queues doesn't mean you should do it. Most are just using 4 or 5.
Enterprise QoS SRND, Version 3.3, November 2005
http://www.cisco.com/univercd/cc/td/doc/solution/esm/qossrnd.pdfhope this helps,
-t
