I don't know about other airlines, but on WN I've recently been buying 2 one-way tickets. I had problems the last time I bought roundtrip with the return flight being on the same day. Always get the same price I would if I bought roundtrip, then again, I don't know if WN always uses roundtrip pricing in their advertising.

I recently purchased roundtrip on DL, and purchasing a round trip = the price of 1 one-way. Not sure how they worked out, but based off their 1-way pricing, the flight home was basically free.