Listing your departure city is key. It's too early to purchase your tickets because that's a Southwest-compete route and Southwest is not yet selling July travel. They should be any time now, though, so my suggestion is that you subscribe to their weekly notification of Internet specials because they may put that route on, in which case, you could get a fare much lower than what is available today.
For example, the last time Southwest put that route on an Internet special it was $61 each way plus tax, or about $130 roundtrip. Compare that to the regular advance purchase fare of about $190.
I think waiting until May is too late. The likelihood of getting the lowest fare on your preferred flights is diminished if you wait until then. I would finalize my plans no later than mid-March. As was previously said, if the fare goes down you can get a credit for future use on that airline, but at least with Southwest, the credit must be used no later than one year from when the ticket was originally purchased. But the good thing about Southwest is that credit money is fully transferable. You can sell it to anyone. That does not violate any Southwest rule.