I only half-heartedly agree with the maxim: distrust .com and trust .org, .gov, and .edu. Why? The rule is often used in exclusion of other more meaningful evaluative criteria http://qcc.mass.libguides.com/content.php?pid=131553&sid=1128167.
You'll notice we don't even include anything about a web site's domain (.com, .org, .edu, etc.) on our guide. This is because the domain is not the most effective criteria for evaluating web sites. Yes, .com addresses are commercial web sites which for the most part are trying to sell you stuff, but there are plenty of .com pages offering authoritative information. Similarly, while sites on .org, .gov, and .edu domains are not commercial in nature, there are lousy pages on these domains.
Rather than asking the question is .com good/bad, consider the sum of the other evaluative criteria: Authority/Credibility; Accuracy; Scope/Relevance; Currency/Date; Objectivity/Bias/Reliability; and Style/Functionality.
Matt Bejune
Coordinator of Library Reference and Instruction