You can. Usually, it's good to have legitimately tested performance reasons for doing so.
Originally Posted by Undefetter
I just did a couple of short tests using the SimC T16H BiS list. The default priority list results in 488,637.8 DPS. Tweaking the priority list to use EM after popping Ascendance results in 490,966.0 DPS. The 2,328.2 DPS difference accounts for about a 0.48% DPS increase.
However, running the same tests with my own gear, the default list results in 449,937.1 DPS and the tweaked list shows 449,991.2 DPS. It's not a statistically significant number.
I think the most accurate answer at this point, assuming you have very good gear that is well-upgraded, is that it is just a matter of personal preference between using EM+Asc or EM+FET. One could make further tweaks to the SimC action list, or try different gearing priorities, and potentially get a different answer.
If you want to replicate this, just replace the three "actions+=/elemental_mastery" lines with the following single line: