Grid time constant is associated with transient faults.
Symmetrical faults have no transients - so it only has an AC component.
For asymmetrical faults (with transient) - it will have both an AC component and DC component. The DC component is an exponential delay which depends on the grid time constant or the X/R ratio. This is where the transients are accounted for.
In older applications where electromechanical relays are used, you will see that CTs are typically rated higher than the maximum symmetrical fault (basically CT rating accounts for transients or the DC component of the grid constant). Why? because electromechanical relays operate on the fundamental frequency. Therefore, they are susceptible to transients in faults. So CT rating must be accounted for that so they will not saturate for assymetrical faults.
Microprocessor relays typically filter out the DC component. So CTs which are connected to microprocessor relays do not need to be rated to see assymetrical faults.
So to answer your question - it really depends... if your X/R ratio is really high because you're close to a generator, you might want to have your CTs rated for assymetrical fault current. If your X/R ratio is low, you're fine with a certain margin above the maximum symmetrical fault current.
Checkout this video tutorial on current transformers:
http://gpac.link/1ETRYuACheck out this one on CT model:
http://gpac.link/1Hp6iBg
