This may not be as directly useful as before, since boss "big hits" now seem to be broken up into smaller chunks with a chance for Defense to avoid each sub-piece, thereby smoothing the damage out more -- so a more complex method (or a simulator) may be needed.
It depends. There are still quite a few big hits like Terminate that land on tanks in one, massive, discreet chunk. And spikiness isn't just a measure of how variable damage is across a single attack. I'm doing some of my own math to calculate spikiness by determining the chance weighted variance of the comparative mitigation factors of the various mitigation mechanisms for each tank and, no matter how you figure it, Shadows are *stupidly* spiky at the moment: you have a ~34% chance to take ~45% more damage than a VG/Guard from an individual attack (the difference is even worse for F/T attacks where a Shadows higher Defense rating isn't a factor and Shadows end up taking more damage in *every* scenario than Guard/VGs because Shield/Abs aren't high enough to offset the *massive* DR advantage that Guard/VGs get). A 34% chance for 45% more damage, even with damage distributed across multiple attacks, is still going to punch through your mitigation often enough for it to be a problem.