It's important, when you use sources, to consider where they came from. You should cross check sources, whenever possible. Comparing sources not only helps you to test their validity, it also helps you to understand your sources in context. Each will contribute something new to your understanding of the event, and to your sense of how you might best represent or analyze it. Be concerned about the biases of your sources.
All writers have biases.
It's important to understand and to interpret them. Consider how a Polish observer's report on the German invasion might differ from a German's report. Often, the history of a source's biases is as significant to interpret as the narrative itself. When reading, pay attention to footnotes. Students who are new to History sometimes don't understand that good historical writing depends on its sources.