A very very very few intact boars are oversexed and will go utterly crazy trying to get to nearby sows. They also make horrible boar stank, and tend to "glue" (ejaculate on) their cagemates or toys. 99.9999999% of boars are reasonably hormonal and will only glue occasionally. Testicle cancer is almost unheard of in guinea pigs.
Neutering has only slight health benefits: the boar is at lower risk for developing impaction problems, which is really minor. Neutering will also protect the boar's friends and toys from the indignity of having glue cut out of their hair. And of course, three weeks after neutering, the boar is sterile.
However, neutering is surgery, and has health risks. Besides the anesthesia risk, hernias are a major risk of neutering. The testicles help block the inguinal ring, and if it's not properly closed during surgery, the intestines can fall into the scrotum. Also, the scrotum area is not very clean, because guinea pigs poop a lot. That can lead to infections. Some guinea pigs are allergic to the most common sutures. Infections or suture reactions can lead to giant abscesses.
When I had Jackie neutered (he was the only oversexed boar I've ever known of, and I was neutering him so he could live with his mom and her friend after his brother Einstein died) I explained to the vet that his mother Suzi had had severe reactions to the sutures. The vet only did abdominal neuters, and he agreed to use a less common suture type, Vicryl. With the abdominal incision, there is the risk of abdominal wall hernia. However, filthy stinky Jackie had no infection or reaction or hernia, and healed perfectly. Then I discovered that the neuter had made him less of a jerk, and he only had the sex drive of an ordinary boar.