Well, you are coming when most shows do great business and will most likely not be offering discounts. So, if you're going to a show that will likely sell out, best to get your ticket ASAP. For shows that may not have discounts and may not still sell out, then you'll be in the TKTS line, most likely.
Pay close attention to the discounts, though, especially the fine print. Many discounts may have December availability, but you sometimes have to buy them by a specific date way earlier than that, like this month.