Unless you're using something like 360 Vault to store photos and videos, people you lend your iPhone to can easily browse through the gallery without you knowing. It makes me anxious just thinking about it.
That's why I don't really like anyone using my iPhone, which leads me to make up excuses. When asked by family and friends, my number one reason for not giving them my device is that my battery's low, but if they were to take a quick look, they'd quickly know I was lying.
However, with a quick Cydia tweak, proof of a low battery is easy, even if your iPhone is completely charged.