I say it depends on the headphones/speakers you have, and if your motherboard's on-board audio is good enough. If you already have the PC and headphones/speakers, I'd say test it to see if it's working good. If not, maybe then you'll want a sound card.
I personally run 250 ohm headphones, and my motherboard audio just can't do it well, so I went with a sound card that had a built-in amp. Works well, and has noticeably less noise than my motherboard.