No problem with rules 1 (its, it's) and 4 (pronouns never use apostrophe). Re rule 2, most contractions can be confused with other words, and look very wrong, if the apostrophe is omitted, but a few (e.g. youre, mustnt) still look OK. Re rule 3, some plural possessive nouns look OK without the apostrophe (e.g. "horses hooves" for "horses' hooves"). I'd never skip the apostrophe for a singular noun though (e.g. horse's hooves). Re rule 5 (Plurals never use apostrophe), when the word is an acronym or capitalized, I'd put an apostrophe in (e.g. the four B52's, I've received 7 RSVP's so far).