It's highly unlikely you'll damage your monitor if you select a resolution listed in it's EDID.
Damaging monitors was an issue with CRTs some 30+ years ago if you exceeded the specs - later CRTs were clever enough to check the signal and show a warning in that case.
It's quite common that computer monitors with HDMI inputs also support SD resolutions so you can hook up eg a DVD player.
BUT: computer monitors often have rather poor scalers, so in that case you might get better picture quality by letting LE do the SD->HD upscaling - I see NUC mentioned in your sig, that should be able to do high quality scaling.
In that case only enable the native resolution of your monitor in the whitelist at all refresh rates (except 25 and 29.97/30Hz as mentioned in the wiki) or the commonly used higher consumer resolutions (eg 1920x1080 and maybe 1280x720). If you use eg a 1920x1200 monitor it may only support 60Hz at the native resolution but offer 1920x1080 at 50 and 59.9x Hz as well.
So, you'll have to check yourself which combination gives the best picture results.
so long,
Hias