Le's face it. Opera singing is not a light, relaxed activity. No matter how good you are, there is going to be some tension required. It will never feel like sitting on the couch watching TV. The issue is eliminating the wrong tensions, maintaining only the necessary ones.
In general, the wrong kind of tensions are automatically reduced by strengthening the muscles involved in the right kind of tension. I used to clench my hands terribly while singing. Try as I might I couldn't fix it. At some point it just stopped. I don't even remember when it happened--because I wasn't thinking about my hands, I was just singing better.
Think about an archer, for example. Before he can draw the bow completely, he must become strong enough to do so. Until then, his shooting is going to be erratic and lacking in power. As he tries as hard as he possibly can to draw the bow his whole body gets tense and trembles. At some point he will be barely strong enough to draw the bow, but still his arm will tend to shake and he will miss, though not as frequently as before. And now the power is there. But eventually he will be strong enough to draw the bow easily. At this point the rest of his body will begin to visibly relax. Telling him to relax before this point is counterproductive, as he might end up relaxing those muscles that are supposed to be working to draw the bow.
And so it is with singing. Yes, work to eliminate tension. But remember that bad tension is a consequence of lack of the correct kind of tension. You cannot relax your jaw while singing until you gain the strength and flexibility to form vowels farther back in the pharynx. You cannot relax your neck and shoulders on a high note until you have strong enough breathing and laryngeal musculature to produce the note in a balanced way. By all means, try to stay relaxed. But don't throw out the baby with the bathwater. Sometimes it is better to carry some tension in unnecessary areas of your body than to drop the required tension in the fundamental vocal mechanism.
