Deciding whether to tell strangers you are traveling alone

When someone at a café or on a tour asks whether I'm traveling alone, I usually answer honestly. A few people have warned me never to say that to strangers, which seems overly cautious in ordinary situations. I can imagine being more guarded when someone is intrusive or asking about my accommodation. Do you have a practical rule for what personal details to share casually while traveling, without assuming everyone is a threat?