They were changed at one point to pulse on a server clock. And have been changed back, for quite a while now.. at least several months.

They pulse every 3 seconds I believe. So divide the number of shammies by 3, to get the optimal time between staggered Tremor drops.

I just have my Call of Whatever spell, which drops four totems for each shammy, and have that staggered. Most of my teams have two or three shammies, or none. So it's two presses over three seconds or three presses over three seconds, for the totem stagger. Then assorted stuff, such as everyone does a Grounding or round-robin for Fire Elementals or Earth Bind etc.