If you don't set up alerts, keep checking to see if seats open up and be sure to check in at the 24 hr mark and then keep checking after that. Silvers get free Economy Plus at check in and some of them will be moving up. And since not everyone checks in right at T-24 hours, a few more better seats might open up. But unless there are only middles available (i.e., can't get worse) you should always choose some seat so you don't end up in a middle/worst case scenario.