It's not unreasonable to believe in conspiracy theories based around 3-letter agencies. The kind of conspiracy theories that one shouldn't give time of day to are the ones that involve an unprecedented level of cooperation between multiple groups of people not known to have the discipline to keep things secret.

If your conspiracy theory involves "most scientists" or "a large number of non-aligned nation states" then it's probably crazy talk. If however it just involves security agencies and small groups of powerful people then it's might be worth considering.