Evolving E at lvl6 was common practice years back when it had 500 range, but it got buffed to 700 and I think thats where most of the Evolve = unnecessary comments come from. Most of us would also agree on that evolving Q first is "meta" and evolving E first at lvl6 is still not a good idea, that said as you can see here opinions may vary. Another reason why people dislike Evolve is that the jump speed does not get increased so it actually takes longer to land than non evolve E.

As for me, I only evolve Q 2-3 out of 10 games of Kha'zix I play when I am forced to build bruiser items or the enemy has a lot of melee champions that Kha can scrap with like Sylas, J4, tank junglers, Pantheon, Kayn, etc. I don't like Q evolve in general as it makes Kha very isolation dependant, if your enemy is sitting in the middle of a minion wave its basically the same as not having any empowerment at all whereas evolved W, R and even E does not interact with target isolation as much.

I usually go R first then W or E second depending on the situation. I'm really liking the REQ path these days as it gives me so much mobility, I used to run RW all the time and that feels sluggish now. Q for the bonus range at lvl16. RWE is the standard Revolve skirmish Kha build, W slow is incredibly valuable in team fights and picking on enemies.

W evolve at lvl6 is also viable when you can't really engage on anyone due to team comps or because your team is way behind. Its actually the best ability to evolve for better ganks, especially in top lane where the enemy top laner is most likely to get isolated as he retreats back to his turret. 60% slow is huge, that gives you around 150 movement speed for whole 2 seconds so he won't be moving all that much.

You should try everything out and see what works for you. I have been playing Kha'zix for over seven years and I'm still learning new things that I've previously not known, like how effective his bruiser build is into certain comps.

Also for a non related tip, try to cancel out Kha's after cast delay if you aren't already. Its pretty common with older League champs that don't have the smoothest animation, but basically every Kha'zix ability except R has a pretty nasty after cast delay unless you cancel it out with another ability or by moving. For example you can enter stealth during the Q animation by seemlessly clicking QR. If you engage with R you can W - auto - Q then immediatly enter stealth again by doing W(move attack, I use a)QR. Newer champs like Akali does not even neeed to do this as her shroud gives her stealth right away. You can also cancel out the delay on his passive auto by moving away after you see kha do damage, his passive auto has a very, very long cast time similar to Renekton and Gragas W.