I agree that this is generally the case; however, JetBlue has a policy for Airports crewmembers to "prep" flights in advance. Part of this process is assigning seats to customers who have no assignment, so if this policy is being followed in that city, the seat map should be a pretty good indicator (aside from blocked seats, which will show as unavailable even if they are open).

In general, if at least 9 seats are unsold, it's my experience that revenue standbys will have no problem getting on. Rarely does a flight have that many revenue standbys, and since you'll be at the top, 9 open seats is definitely a good thing to hear.