When is it appropriate to use a biased opinion in a research paper

All sources will have some element of bias in them due to the fact that they are products of a specific person's (or specific people's) views. The best ways to deal with bias are 1) to acknowledge the bias and comment on it, 2) to include several sources with differing biases and discuss their differences, or 3) the analyze what the existence of the bias might indicate about the issue.