My background
I've actually taken a few psych classes and I'll admit they have helped a bit.
Going back to what AlienMindsInc said the DSM does say a lot of negative things about sadomasochism and I actually did a project on that topic and how the idea of paraphilias does not necessarily imply a disorder but simply a preference. Though not always socially acceptable and sometimes they may be considered a disorder, in the case of pedophilia and such, they are simply a sexual preference.
That being said I don't believe that having a psych degree would deter any true Dominant as they would see rationally the same conclusions as I have drawn in the above paragraph. Multiple psychologists have already written articles on the subject that certain paraphilias should be removed from the DSM as they are not harmful or cause for alarm.
As for me personally my psych classes have helped me a lot in my role as a Dom. While the example Isabell gave does involve head games it doesn't take a psych class to know that you can freak out a blindfolded sub with a threat of a knife. However, taking a psych class can help you watch the reactions of the sub during that threat. Run your hand across their chest when you make the threat, not for the physical sensation but to check their heart beat. If you say knife to someone blindfolded their heartbeat would normally get faster, see if you feel that, if you don't if they act scared but their heart remains the same you know they're just putting on a show for you in their actions and you need to step it up if you really want to get their blood going but also it tells you that you have their absolute trust. You gain that trust and you can manipulate it to your advantage in future situations. Take a class on Freudian psychology learn about the various stages of human development. The oral, anal, phallic, etc. Use those for your advantage. Learn to discretely monitor vital signs, (breathing, heart rate, eye motion, shakes, clamy skin, etc). Then monitor them when doing various things and test the reactions. Your sub may say they "hate anal but only do it to please you" but their body may betray them and say otherwise that they love it, taking a course in Freudian psych would show that this is due to underdevelopment in certain stages of life and cause for a fixation on it. Little things like this can always help.
So no I don't think it's necessary to have a degree in psychology to be a Dom, but I won't lie it certainly helps A LOT. Take a few classes, read a few books. You'll be amazed at how it helps out.
Also I suggest learning a little about criminal interrogation techniques. Not necessarily to do an interrogation scene with your sub but by learning them you can read your subs reactions a lot better. My school has a good criminal justice program and I've taken a few classes as electives and learned a lot.