False. In a particular period of history, diaries journals, and letters can be very trustworthy. We actually get a lot of information about historic events from letters, diaries, and journals... and since those people lived in that time, we can get a view of what it was like, what people thought about it, etc. And even if the information was biased, you can compare what they said to what historians and others believed in that time as well. And, unless it was a child, not all of the adults are 'imaginative'. Some genuinely write about their lives and experiences.
I hope this helps!
~kaikers