It seems safe. It took me a few tries only because I wasn't paying attention to my phone.
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It didn't trip Knox for me. But I had to install a program called safestrap. To be able to install a custom rom. I haven't read up 100% on safe strap. But i believe it's the only way to get a custom rom without tripping Knox on a non dev unit.
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Beware: Kingo might be sketchy. And why wouldn't a free root service be suspect? See androidnews.co.in/2013/11/08/psa-use-kingo-root-vroot-root-android-device/