I play a few styles of music and practice to a metronome and a click. One of the practices is dragging or pushing the beat on purpose for the sense of feel and so on but still while keeping good time. In fact on some R & B dates, they will look at you very funny if you don't lay back full time, though they want you to have perfect everything. I don't hear about this much in thrashier metal, death, or speed. I'm sure when a groove section, or clean section comes up, someone in the band/production is looking for a bit more of a open feel and it's probably cause they want the snare or someone else to lay back a bit more. You'll find the best bands of the genre you like study other style and pick up these feels in put them into metal. Just like some lead players infusing jazz or classical, some infuse timing ideas.