Google Voice doesn't come stock with Android. You can install it if you have a Google Voice account. If so, you probably know which one you want to use for texting. For example, I'm on Sprint and have GV integration enabled, which means my GV number gets used for caller ID and voicemail instead of my Sprint number. If I use the stock messaging app (or any other like Handcent), the texts go out as SMS through Sprint, with my Sprint number. If I use the GV app, the texts go out using the internet (3G or Wifi) and have my GV number. I never use the built-in messaging app. I'm pretty sure anyone who installs GV understands the concepts behind it and chooses to use either GV or SMS for text messaging without any confusion.