Depends on what you want to do after you get a PharmD. In the not uncommon chance you decide you want to switch gears and apply to medical school after completing pharmacy school, practically all medical schools require you have a bachelor's degree, and only some will waive that requirement for someone who has a PharmD only. I have yet to hear of a graduate program (not medical school) that won't waive the bachelor's degree requirement for someone with a PharmD only, but I have heard that graduate programs that do not have experience with applicants who have a PharmD only usually need some convincing / don't have an administrative process built-in to their system, and need to create one. This is usually a non-issue, but can be an additional hurdle.
Some pharmacy schools recognize this problem, and have a BS program built into their PharmD program. The usual set up is students who don't have a BS degree can petition to get one after they complete the first couple of years of the PharmD curriculum. It might be worth inquiring if your pharmacy school has such a program, and consider taking advantage of it if you want to make it easier to jump ship from the pharmacy profession down the road. In terms of pharmacist-specific jobs (i.e. jobs that require a pharmacist license), having a bachelor's in addition to a PharmD doesn't give you any edge. Maybe if you majored in something unique or eccentric it can serve as a conversation starter and might make you stand out a bit from the crowd, but that's a pretty insignificant advantage overall.
In short, getting a bachelor's will make it easier to get into medical school or other graduate programs, and it provides a bit of a safety net if you end up deciding not to finish pharmacy school.
