Yeah, it's a commonly reported observation that stuttering requires a certain stress threshold before it manifests noticeably. Many people don't stutter speaking to animals, toddlers, or when speaking aloud to themselves, as you noted. Personally, I don't seem to stutter while reading, even when reading out loud to an audience. Not even on tricky words that start with vowels like 'a' or 'i'. It must be related to the stress/effort threshold thing.