For a genuine bona fide spoiler I think the answer is, 'it's never alright', but what counts as a spoiler?
Everybody's all hyped up on Reddit about the new GTA game due in a few week's time. Some details have leaked, including things like which songs will feature in it, and what sort of items your characters will need to collect to complete the game, etc. To some, these are massive spoilers and they have had a little meltdown after waiting two years since the game was first announced. To me, that's no big deal. Somehow, even if the entire premise and story in great detail were leaked about GTA V, it wouldn't really affect my enjoyment much. Games are for playing, not watching. Maybe that's from growing up on games like Pitfall and Pac-Man that didn't really have stories. In a way, when I see people acting serious about the 'story' in a game, I am reminded of how in 1992 I thought Guns n' Roses should be taken seriously now because there was strings in November Rain.
The worst spoilers I can imagine would be your long-running serials on TV, like your Boardwalk Empires, The Wire, etc. It's better to never even mention those kind of shows on the internet. One push of the penis and your enjoyment is over. That unfortunate scenario has happened to me several times.