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Hmm, I recall (?) Cisco used to use a default Tc of 25ms, and I also recall (?) newer IOS implementations changed the default to 4ms (this, I suspect, to better support VoIP).
Setting Bc to obtain an optimal Tc, depends on what your service needs are. If you're shaping to deal with a downstream bottleneck, and your doing QoS, traffic with tight timing requirements, generally a smaller Bc (and Tc) is required. This because, as you minimize Bc (and Tc), the shaper performs closer to a physical link of similar bandwidth. However, I believe this adds to processing load, so you want to avoid needlessly doing this. Personally, I either just go with the default or, if I set Bc, I've found a 10ms Tc usually works well.
BTW, when working with a rate limiter (or policer), as they don't queue over rate traffic, using a small Bc (and Tc) tends to drop (bursty) traffic.