I don't know if it is proper or not, Jeff Cooper uses the high thumb hold, others don't. I have shot both with my thumb on the safety and below it, I tend to use the high thumb hold more, I guess.
As to the high thumb hold interfering with slide travel, I guess it would depend on a couple of things, like do you keep both thumbs resting one the thumb safety lever or how high the lever is on the safety. Several parts makers offer thumb safeties with thumb shields to prevent lateral pressure on the slide. And there is at least one parts maker that offers a low extended lever thumb safety, where the lever is low enough that your thumb would be in the same position as if you were not resting it on the lever with a standard thumb safety.
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Ne Conjuge Nobiscum
"If there be treachery, let there be jehad!"