Mass confusion about present and future: Hangouts, SMS, MMS, and Google Voice
If you keep using the Google Voice app for texting, it doesn't matter if you activate SMS via Hangouts.  However, any texts you send out of Hangouts will go through your Verizon number, incurring carrier charges.

If you are running CyanogenMod 11 ROM, you may want to look into Voice+.  It's a custom app that allows you to pipe all "regular" SMS through Google Voice, from any app such as Hangouts, Messaging, HandCent, Whatsapp, etc.  It's still in beta (as is the entire CM11) but works quite well for keeping all the messaging together in a single app.

When Hangouts got ability to send/receive SMS last year, Google promised that they will eventually merge Google Voice functionality into it, thus making it a single app.  This looks to be slowly happening behind the curtains. In the latest Hangouts v2.1 update, bits and pieces pointing to Google Voice integration started showing up.  We will most likely hear more about this in the upcoming Google I/O developers conference this coming Summer.