While it will depend on how each individual raid decides to attack certain encounters, the short answer is this: there's really no reason with the 2PT16 to not have SD up pretty much at all times while actively tanking. Even fights where FR usage will likely be more common, SD uptime is just as important or more important (Malkork is a pretty good example of how both SD and FR are really important, where avoidance slows down our damage taken debuff stack and FR heals will keep our absorb bubbles very high). Also, 4PT16 can actually overshadow some FR usage in terms of total healing you do to yourself, but situations calling for FR will be very recognizable and will allow you reaction time to use it.
I should make a side note, though. Part of the reason why we'll have ample reaction time is the stamina modifier boost and the reduced CD on Barkskin (tanking LMG helps, too). Our "passive" EH is very good to where we can eat large attacks that may have required gear modification or more stringent cooldown usage on live.