I recall that there are minimum Tc values, although how small might vary per platform.
Although @Elliot Dierksen is correct that small Tcs, make for more precise ("precise" as in more closely being like a physical link for that "speed" in small time intervals) shaping, however, I recall (?) going "too small" can bump into the issue where the Bc is smaller than some possible packet sizes (which results in those packets always being consider out-of-contract (or overrate).
Unless you have a real need for a very small Tc (Elliot's mention of VoIP is a good example, including his mention of using 10ms [which, is often what I used when working with VoIP]), a larger Tc might (slightly?) reduce resource consumption (mostly on software based devices?) and, for longer term averages, a larger Tc will still provide the same shaped rate (just, possibly, more bursty).
I.e. don't reduce Tc just because you can, but use what you really need.
BTW, often overlooked, software based (e.g. ASR?) QoS doesn't engage until the interface TX queue overflows (which is only FIFO), so often, you need to set tx-ring-limit to a smaller value than its default. If you need a small Tc value, you'll probably also need to manage the TX buffer size too.
