I'm annoyed with the etiquette advice I've found on the knot and other sites that say one should NOT include a deceased parent's name on a wedding invitation. WTF is that? I understand that that is not what's normally done, but that doesn't mean it's improper to do it. Whoever writes these articles clearly does not have a deceased parent. I think it's perfectly appropriate to include (and honor!) the parent by writing, "the late" before their name on the invite -- I saw this advice on one of the web sites I found while researching this topic.
People's feelings and a couple's unique situation should come before etiquette rules, I think.