Saw in some traders' screenrecordings of them checking the level roughly 10 points above/below from what they apparently deem to be an existing S/R. Some of those being whole-number figures.

Or they'll apparently eyeball the size of the current swing and click the mouse on the level mark one magnitude of this swing above/below.

Are these actually established levels played off of by many?

Hi,

projecting the size of a leg down from a resistance upwards upon the break of it is a common thing to do (sometimes called "measured move").

And some traders do pay particular attention to whole numbers.

But perhaps you can add some context to your question as both techniques can only have value when considered among other signals / technical tools.

Regards!