Usually when I size boots I go smaller. I wouldn't try on your boots with the thickest pair of socks you have, because the boot is going to blow out as it gets worn. When I'm trying on boots, I go with what is comfortably tight with just liners because then I know that when my boot does blow out, socks can help me keep the boot fitting longer. It really all depends on the manufacturer, as I am sure you know no one sizes their shoes the same way. Just make sure that your entire foot has good contact with the boot and that there aren't any uncomfortable pressure points or spaces between your foot and the boot. Ensure there isn't any heel lift and that while you can feel your toes at the end of the boot, you can still wiggle them.