typically on every gear set i'm looking for a specific stat that matches the gear or is speed. so for perception i need speed or accuracy. if it's savage, i need crit dmg, crit rate, speed, or atk%. if it's crit rate i need a crit rate/crit dmg or speed roll, etc.
only exception to the speed rule is stoneskin typically.
if its a really good lego piece i'll take it to 8 to see. if it's epic i'll do 4. if it's blue i sell unless i'm desperate for that gear set.
that all depends on where you are in the game though, if you have 800 open slots then you might be less picky. if you have less than a 100 then you're looking for excuses to sell, for example if the speed is only 4 to start with, its gone unless everything else is amazing.