Either way you approach voiceover, you need training

Backstage has a great article on whether voiceover classes are necessary.

It's a great read, and Dale Balestrero has some great insights and explanations relating to why actors need voiceover training (I'm somewhat curious as to why he doesn't make more of a point to mention that voice actors are actors... hence the title; it's a nitpick, but one that I think needs to be made more often). In short, Balestrero points out that one's acting skills are very important to voice over, however, the medium is not the same as more traditional forms of acting. Therefore, it does the actor well to know more about voiceover prior to attempting to make a career out of it.

On that same front, one of the biggest problems I see with new voiceover talent is that they train for voiceover, but not for anything else. Just as I stated earlier, voice actors are actors. It's as plain and simple as that. Regardless of where your specialty lies, if you want to be successful in acting, or in any field for that matter, then you need to know a wide range of skills (which may or may not be directly related to your chosen field) so that you can be a more versatile practitioner.

It is this versatility which allows most people to find new ways to succeed, even if they've already found a thousand ways to previously fail, and for this reason it should be the lifeblood of every actor, regardless of their chosen medium.