Yes, and this means a lot when you are discussing what talents or gear to use. For example, I did not use CoW for any of the encounters on heroic, but normal was such a snooze fest that I ended up with the talent on several encounters just to give myself something to do. I was also using DPS trinkets and a few non-spirit pieces for the first 10 bosses, which I did not do in heroic. Anyway, the real reason why I am requesting the distinction is that I would put more weight into any suggestion if it was intended for my difficulty.
I know that you were doing Mythic last tier. Well imagine if an LFR player told you to use CoW on Gruul because it increased his HPS. You would probably just ignore his suggestion and look for something that applied to your level of raiding. Well now imagine if he did not tell you that it was LFR. You might think that he is crazy, or if you were not really familiar with the encounter, you might start to question what you are doing. This is the feeling that I try to avoid. There are set number of people whom I already trust on these forums, but whenever a comment comes from a stranger, I would prefer to know what content he/she is doing.
Now I realize that the difference between LFR and Mythic vs. Normal and Heroic is much more significant, but I would really appreciate if we noted the difficulty for the reason mentioned above.