I talked to the TSA people one day when there was no line and they told me that it's usually dead at the NW/DL/UA/CO security gate except between 4:00 and 5:30 M-F. When I've been there for the 1:30 or 3:30 PHL flights, I've never had to wait more than a minute or two. When I had the 5:30 PHL, the line snaked around all over the place upstairs and they were holding people downstairs.

The WN security gate seems to have rushes during their major flight departure times, but none so much as I saw one Sunday (Oct 2001) when the line went through all the corrals upstairs, down the escalator, across both the FL and WN ticket counters, and then back down the hall part of the way to baggage claim 1/2/3.

I usually look and if there hasn't been any holding at the bottom of the stairs next to the ticket counter, I take that one. I don't usually fly high traffic times, and rarely get stuck in any traffic at the checkpoints.