Now that Air Tran flies from STL the B checkpoint is not the magic it once was. Generally it's still better that than the C/D lines, but with a 06:00 departure and fewer lanes it can be backed-up.
True. You basically have to walk past the entrance to the "C" lines anyways (at the bottom of the escalator, once you head down a level from the ticket counters), so it's worth it to just pause there & ask the TSA at the head of the line...he/she would usually know. You get a really, really good scope of the C lines as you're going down the escalator, and can see if there's a crowd, if the "elite" line is open, etc.
IIRC, they're re-doing the customs facilities at STL to better accomodate the carriers going to Mexico, etc. They've been using the "old" TWA ones in C, but apparently, once the "new" ones (in either D or E, I think) are done, the "crunch" on the C lines may not be as bad.