I have learned to use key a little less as an absolute.
The way I see key is mainly by the type of track.
If it is a track heavy with drums and less melodic content. I almost ignore the key when mixing. Because the melody or vocals almost never clash or sound bad. Because there really is none. Types of genres that are "simple" like this are, Techno, Tech House, Bass House, and even a lot of Hip Hop or Garage. A person could mix almost any of theses mentioned genres with never looking at the key.
The tracks I tend to be careful with when it comes to key. Are genres. Like Mainstage, anything with heavy vocals, like Camelphat, Melodic House, Pop Music, older or rock music. Etc. Of course any type of careful remix needs a lot of attention to key.
But, even then depending on how a Dj set is done you could easily skip key matching and just blend drums or a non melodic part to avoid key clashes.
My girlfriend mixes very popular music almost seamlessly without paying any attention to key. But she always mixes intros that have simple drumbeats. By the time the unmatched vocal or melody comes in the other is gone and nothing clashes.
But yes, it is much easier to always mix in key and have a good mix.
