is it safe to use palmguard and keyboard cover?

I heard Apple's warning about using palmguard on MacBook Pro retina display can damage the retina display. I would like to know that can it happen no MacBook 12 inch, or not? I also plan to use keyboard cover but I still worry about this topic too. If I using a hard case, can it solve this problem? (I thought when using a hard case the gap between display and keyboard will wider) or Screen Protector is the answer?