The bottom rail is what the head bolts (or through bolts) bolt into, it doesn't fail but the uprated part is stronger so the head/block/rail assembly is stronger and the head less likely to move around on the block.
You don't really need to renew the head bolts unless they have stretched - do a search for the measurements, I understand the OE head bolts are a good spec and rarely fail.
No need to skim the head if its not warped and the head face is not damaged, all you would be doing is skiming a perfect head face to achieve a perfect head face.
Has the HG been changed before? if so I would tend not to bother replacing it unless it fails but I guess doing the work yourself its a small outlay for greater peace of mind