As far as JetBlue goes, they operate on 100% bag match.

Also, I flew them BTV-JFK-SEA a few weeks ago and found the security to be very tight. Both in BTV and in JFK the airline staff was wanding lots of passengers as they boarded, and the whole operation was still running with tight turns, full planes, and on time.

I had a 3 hour layover, so I left the terminal, and was surprised that the security line was less than 10 minutes long, even though there was a large bank of departing flights.

This incident seems to be an anomaly. JetBlue is a well run airline, although I think their inflight service (friendly F/As don't make up for only 2 beverage services on a 6 hour flight) is lousy.