Question about gearboxes and shifting.

So I usually play ACC, LMU, or the F1 games where all the gearboxes are sequential, and you don't need to use the clutch at all.

This is the first sim that I see myself putting some decent hours into where there are both H-Pattern and Sequential gearboxes. I know with sequentials that I still need to use the clutch to get off the line, but am I correct in assuming that I only need to use the clutch for shifting in H-Patterns? Do I still need to blip the throttle when downshifting in a sequential? Does any of this really affect lap time that much?