Some of the cases when you should consider taking a second medical opinion:
There are times when a patient is told by his/ her doctor that further medication or treatment is not going to benefit his condition, and there is no way to avoid the consequences. This is a case that definitely calls for a second opinion. You have nothing to lose, but a hope to gain hugely if your doctor was in error, and a second opinion could save your life.
If your tumor is almost operable, but not quite, or conversely almost inoperable but not quite, you might want a second opinion. You might either save yourself unnecessary surgery or you might save your life. Similarly if your tumor is on the borderline of requiring adjuvant chemotherapy you might want a 2nd opinion.
If you live in a rural are or a small town, it is worth taking a second opinion, either online or in person, from a doctor of a major hospital. We are not to say that the rural areas may not have good treatment or qualified doctors, but the probability of the doctor seeing a cancer case in a thinly populated rural area is much lesser than a doctor of a major hospital who sees large number of cancer patients on a daily basis.
If your doctor wants you to take part in a clinical trial that he is participating in as an investigator, then I think you ought to get a second opinion before signing up. One of the toughest things about running clinical trials is finding patients to participate. So the doctor has an interest in having you sign-up that goes beyond his interest in getting you the best treatment.
If your doctor tells you that you have a rare cancer, then you should definitely consider taking a second opinion. There are chances that you might either not have a rare cancer, or even if you do another doctor might have better experience in dealing with the rare kind of cancer.
Second opinion can benefit everyone, and there is no distinction at all. Read about how cricketing legend and star from India, Yuvraj Singh benefited from getting a second opinion on his cancer to arrive at correct treatment.