The sound from the (tv) screen is sufficient for viewing, for sure. If you're just worried about hearing the tv dialog, etc., no problem. So, if you're considering a sound bar to plug into the tv only (not into a stereo), I wouldn't bother.
I've got my tv hooked up to my stereo in case I'm watching a show or movie where it's best watched with the better sound. But most of the time, I watch with only the tv sound.