If it's the same amount to book one way, as it is to do RT, than book them as OW. I booked DD and BIL on a RT flight and a DING came out for the back 1/2 but I can't change or cancel the return without it cancelling the outbound, and there's no more cheap flights for the outbound. This is the first time this has happened to me, but from now on I plan to make sure it doesn't happen again.