If you're not a native speaker, I think it just makes you look pretentious.
Na, bro, being pretentious makes you look pretentious. If you're trying to impress people with your 1337 Spanish hax, you're going to look like a doofus. Like always, when people talk about stuff they don't understand, as if they understand it. But if you approach such a situation with humor and fun and humility, you'll get a great reaction almost every time.
At least, everywhere I've ever lived...they say people are less polite in big cities, but I haven't spent enough time going to restaurants in big cities to really comment on that. But even if they're not nice, I'd still rather be unpretentious.