Midwest is CASS and has unlimited jumpseaters. You can just show up or call ahead to reservations.
There are a few routes these days that are weight restricted at times due to our crappy MD-80s like....PHX, LAS, DEN (717 too), MCO, TPA. If in doubt ask a res. agent how many seats are available. They're usually pretty good at having an accurate idea of how many seats will be available on these flights.