The provision for starting the S76 with the rotor brake engaged is in the flight manual, so it shouldn't be that dangerous or destructive. The part about not starting with a blade over the engine exhausts is in the manual also, & ours are placarded against it. I don't start with the brake engaged as a normal practice, but I sometimes do offshore in gusty wind conditions.
We also do maintenance runs with the brake engaged, so the mechanics can check things in the engine compartment with the cowling open, one engine running at a time. You can't run it with the blades turning & the engine cowling open, so some leak checks, etc have to be done with the brake engaged. You just have to remember not to turn the brake off.
As for the 206, I once started one on an OAS checkride with the fuel valve off. The checkpilot passed me, saying he had seen me reading the checklist, & I reached up & touched the switch, I just didn't turn it on. I was embarrassed, naturally, but I know I wasn't the first to ever have temporary retreating brain stall.
I like to turn the blade 90 degrees. By the time you check for blades turning at 30%, it should be passing over the front, & it's easy to see it turning. I've also lit one off with the blades tied down, & afterwards always made one last check to make sure the blades were on the side.