I haven't found any major problems lately with the on-board audio output on either the B or the B+, for the most part. Things have much improved here from the early days. Quality is undoubtedly higher with a dedicated DAC, but for talk radio or podcasts or casual music listening, the on-board is fine to my ears (the chosen speakers will make a much bigger impact than the DAC, in my humble opinion.) Try it and see (hear) for yourself.
The fly in the ointment is PulseAudio, it doesn't play nice with the on-board ALSA sound driver.
On the other hand, with the extra USB ports on the B+, I didn't see much point NOT to use a cheap USB DAC, and avoid issues with the on-board sound driver.