I completely agree with Robot and RelativityDrive.
I work for an ISP in the reselling of secure internet to banks. My companies BGP routers and DS3/T3 ( We have 2 OC192 connections as well). We use shaping less than what the SLA is, simply since the ISP gives around about bit rate to connect at.
In the ATC class on the shaping and policing, more specifically Brian McGahan explains the the serialization clock speed can't be exceeded. This is why we need to shape. I'm sure your familiar with the concept. However, since we still use Frame pretty extensively, I'm on a team to migrate 3,700 frame T1s to MPLS over the next 2 years.
If your Tc and your Bc are configured correctly then you should have no problems forwarding with limited retransmissions.
If your Bc is less then it's shaped average, this goes in the "bucket" as Be or excess burst, so on a Tc you can add your "bonus burst bits" to the Tc and these bits will be forwarded. They will be above the Bc rate in the time interval of whatever is configured. But since you have bit time bank they won't be affected or dropped.
One other feature of QoS I thought I would struggle with, at least I think I understand it. Is queueing. If the HW queue is full and your VC hold software queue is say 1500. You are able to buffer to the set 1500 if the HW queue is busy, (depends on the configurations on the interface). If the incoming bits can't go anywhere they get dropped. I could go a level deeper if you have multiple queues availble for different QoS policies. but I digress before that.
All in all, the important part is giving your data, whether voice or other wise the best change to go where there being sent. I deal with clients calling in alot that they are seeing dropped traffic and retransmissions. The fun begins there, I have to figure out what type of traffic and so on. Wireshark has become my best friend. Once I determine what the traffic is I can appropriately classify the traffic.
Another tool I like is NBAR, I am a fan of the PDLM downloads to the flash of the router, makes classification much easier. You can then match protocol if you have NBAR running. I prefer NBAR over IP accounting since it gives you the protocol and top talkers.
Rob Riker, CCIE #50693, CCNP Service Provider, VCP5-DCV