Since all the monks I know complain about TP usage, it is a fairly significant (you gain almost a minute more of full boss uptime) TP efficiency increase to always swap out True/Twin.

Unless you know a disconnect is coming and you need Twin for a Demo/Snap coming up, I would say always alternate, even if it means the damage falling off. You lose roughly 1.4% dps (less than that actually, but i'm lazy) and gain quite a bit of efficiency.

You gain 3% absolute efficiency, but keep in mind TP drain is coming off of margins, so it is closer to something like 20-30% slower TP drain. It makes a huge difference, esp with the amount of relative SS monks have with GL3.