I have never said this to anyone's face before, because how do you tell someone, "Hey dude, you're spelling your name wrong"? Well just like that, I suppose. But we must admit it's awkward for everyone.
Jeremy and I are the Brockways. We are not the Brockway's. If Loki were married, he and his wife (or husband - he kind of gives me a big gay bear vibe) would be the McLardasses not the McLardass' or the McLardass's. Likewise, if Puck were a single father of two demonic children, they'd be the Von Douchebags, not the Von Douchebag's. (Where's the mom, you ask? How the hell would I know? It's not like I sit around thinking up fantasy lives and ridiculous last names for my cats. That would just be pathetic.)
In fact, in the case of nouns, proper or otherwise, apostrophes are never ever used to make plurals. Possessives, yes. Plurals, no. That means, Mr. Coffee Shop Owner, you do not sell smoothie's, you sell smoothies. Unless, of course, you're selling something that belongs to a smoothie, in which case, you're missing a word.
And no, it does not matter whether the word ends with a vowel. I'm looking at you, Panini's. You may have placed my french fries in my sandwich where they belong, but unless you're selling me sandwiches that belong to another sandwich, you're spelling your name wrong.
Also, I just learned Panini's is a local chain that sells paninis (the Italian word for sandwich) and thus you may well have no idea what I'm talking about. Well, they make delicious sandwiches with french fries in them and they spell their name wrong. Ironically, Panini is also the name of an ancient Sanskrit grammarian etymologically unrelated to the sandwich.
It is possible that I carry a Sharpie around to correct such errors when I find them. I'm pretty sure it's not graffiti if by changing it, you make it correct.
In summation, before you order those address labels for the Christmas card list, remember that you aren't the Smith's or the Jablonski's or the Lipshits'; you're also probably not the Smiths or the Jablonskis or the Lipshits. But whatever your last name, I assure you it's not spelled with an apostrophe.