Plate covered is an awful shield for anything but cav. Morningstar especially will rip up a plate covered in a couple hits, and it's slower than molasses to boot. Any shield slower than ~85 speed doesn't allow you to recover from accidentally dropping your block during a feint/hold. I don't notice a difference between 90 speed and 100 speed, but <80 speed is a major liability.
Anyway, I've done 4, 5, and 6 shield skill. Even with a +3 Brown Kite, 4 shield is a bit too low, 5 is about perfect, and 6 seems like overkill most of the time, although your forcefield against ranged starts to get pretty huge at 6 skill. Avoid axemen, they will break your shield fast no matter your skill (unless it's 10+, but then you are gimp for other reasons).