As Irish observed, Southwest Rapid Rewards is a fixed point redemption system where 1.6 cpp equivalent is in the middle to high range for its value. With Southwest, it's more important to ensure you have the best fare, such as a "wanna-get-away" fare.
Airfare is extremely volatile and fluctuates daily. In general, it's never a good idea to purchase (unless fully refundable) more than 4 - 6 weeks before travel. Invariably, prices will narrow and being to lower towards 4 - 6 week mark. Usually, the best fares are those where the Carriers will adjust/compete with their pricing within 3 - 4 weeks. During the 3 - 4 week mark, if you see a good price, then it's best to purchase the ticket. In addition, Tuesdays and Saturdays will have the best pricing for some Carriers; so, if you're flexible, you will always get a better price than others.
To narrow down best pricing:
1. Use google flights - check your fare/route/day for ALL carriers.
2. Check Kayak (or, some other equivalent). Once in a while, they'll have lower fares than published the carriers (though, google flights should include these other airfare search engines).
3. All fares have a 24 hour refund period.
4. On your net cost, do factor in the rebate of any rewards, CC points and promos (search for them).
Finally, Southwest fares DO NOT appear on searches and must be condutcted on their site. Southwest is unique in that they do not charge a fee for refund or exchange making them ideal to use as a hedge. In other words, to ensure you have a flight at your date of travel, reserve Southwest; when their fare lowers, book the lower fare and cancel the existing reservation with a higher fare. Rinse and repeat until T - 6 weeks and start checking the prices for ALL carriers. If another carrier offers a considerably better price than Southwest during the period, reserve the ticket. Then, wait 24 hours to see if Southwest price matches the fare; if not, keep the price and cancel your Southwest for a full refund.
This is how I purchase my tickets. Naturally, do sign up whichever FF carrier used, utilize the perks and the Airline's CC if you plan to fly them frequently.
Good luck!
*Edited* PS - If you have Soutwest booked (as a hedge), you can wait after T - 3 weeks and sometimes score some really good fares! However, generally, within 2 weeks, fares will not lower much and will increase within the 10 day mark.
PS - Also, check round trip fare vs. one way fare X2. Sometimes, one way routes with different carriers will be more cost effective.
