The short answer: Yes, you absolutely do need voice over classes, there's no doubt about it.
…okay maybe that could have been shorter, but here's an even longer answer:
You might have the voice of a golden God or Goddess. You may "get it." You might be an absolute pro at interpreting a script and delivering what the client's looking for, but there will ALWAYS be a vital need for outside direction and critique.
As a musician, let me liken it to music.
When I write music alone, I'm my own judge, jury, and executioner. The input and feedback going into my composition are coming from precisely one source - myself.
When I write music with other people, I cannot put into words how exponentially better my tracks turn out. It is absolutely insane what many minds can accomplish upon collaboration.
You may be thinking, "I thought I was supposed to work on this every day, alone, spending time learning to listen to myself?". Yes, daily practice is critical, and reading proper scripts every day is hugely important, but without outside help, you could be practicing yourself down a hole that may be difficult to climb out of in the coming years of your career. You need more ears, and those ears should know what they're listening to and how to guide you in improving your weaknesses and playing to your strengths.
Someone who spends their entire 9-5 workday listening to, engineering, and directing voice over will undoubtedly hear something that you can't. We're talking about talent agents, audio engineers, and coaches. It's their job to translate what they hear into a valuable direction in hopes that it "clicks" with you.
You're going to grow your voiceover skills tremendously.
Take it from me, and once you've been through enough training, you'll absolutely want to listen back to the early work you did, and it won't even compare with how much you've improved.