One thing you can count on, Benji is not going to be happy having you working with shears or clippers around his anus and genitalia. When you add in the noise of a set of clippers, Benji's anxiety levels are likely to go through the roof. Be sure you take your time and give poor Benji a chance to get used to the tools and your hands working on his derriere if you want the task to go smoothly.
You may have never given much thought to the hair on Benji's backside. Keeping it properly cut and trimmed back is a very important part of the overall grooming process, though. It can also be quite important to Benji's health. Long butt hair can harbor so many parasites and so much fecal matter that it can be bad for Benji and for your family's health.