Absolutely! In some cases, when Dings come up, they're for specific flights, not just for specific dates. By booking one-way, you can reserve whichever flights work for you, without having to cancel a RT ticket (and therefore having to re-book the other flight at a possible higher rate).
To confirm, when I buy tickets for our upcoming May trip online, I should book each way separately? Would the initial price of booking both ways separately be higher priced then booking a RT ticket?
Southwest lists their prices as one-way. Depending on your flight dates, your tickets may be different prices for each way. Definately book one-way only. The pricing does not change weather you book one way or round trip.