In the case of identify-this-game, all anybody is doing is guessing. That's kind of the primary problem of the tag, and it's one of the reasons I (and many others) happen to think it's a terrible class of questions. So no, there's nothing wrong about doing so in those circumstances.
As for other questions that are more concretely answerable: A lot of it is situational. The key is, even if you're conjecturing, you'd better be able to a)back it up, b) hit up The Google and do a sanity check to make sure you're not way off base, and c) provide a 'complete' solution via your conjecture, not just take a stab at the right direction to head.
Here's a pretty good example of an answer that's just taking a stab at things. - There's a pretty good chance he's wrong in this case, but he's basing his conjecture on actual knowledge, pointing out that it is just an idea, and most importantly, even if it's a wrong answer, it's still a complete answer.
So long as both of those are true, go for it - just be sure to note that you're taking an educated guess at things, and be ready for the downvotes if you happen to be wrong.