Here's the way I think of it...and it's not a popular opinion here. While I like a refund or credit as much as the next person, I just can't bother spending a ton of time checking on what the current fares are. Sure, they go down sometimes. But, do I expect a refund? No. Nor do I expect to be billed if the fares go up!!! If you are concerned about getting refunds, then book refundable fares. Yep, they are ridiculously expensive. But, you can get a full refund if need be.
The vast majority of airlines don't give you a credit for lower fares. Sure, you can make try to rebook, but there will be change fees associated with that. So, if your fare doesn't drop at least $100, you're better off just sticking with what you booked.
There are some airlines that will let you get a credit if the fare goes down. JB does it...as does SW. Not sure about Airtran..maybe.