I think people need to take into account genre slightly more when it comes to harmonic mixing, for example a super minimal techno set (style of DJs like Ben Klock, Marcel Dettman, Sven Vath etc) HAS to be harmonically mixed or it just sounds awful. But harmonic mixing does not mean you cannot change key at all, the key wheel (http://www.harmonic-mixing.com/Images/camelotHarmonicMixing.jpg) is incredibly handy and allows you to really learn what key changes you can and can’t do and what impact they all have on the sound of the transition. Some are fairly small impacts but some like going up or down a “5th” can have a really large impact on the sound and create quite a euphoric or incredibly dark sounding change. In regards to BPM I tend to have a +/- 3% rule but as the previous comment mentioned this is just down to personal preference, but bear in mind that if you’re using key lock and try to change the bpm of a track too much the sound may start to distort. A handy trick for big bpm shifts I tend to find is to use tracks that have no beat in the intro, but even then doing this too much can create quite a stressful sound.
Hope something in here was helpful, the information would be way more specific if it wasn’t 2 in the morning!
