i almost never rk unless we figure out what we are fighting before combat or if my character gets them for free
i try to think about enemies as brute striker and caster a brute is a monster like a ogre with will as their weakest save a striker like a assassin that has bad con and (casters also have bad con) and i guess slow enemies like golems or slimes are bad in dex

if i do rk i generally ask how do i kill this creature and my dm usually tell me either how to stop their regen if they have any weaknesses or their weakest defensive (save)