It depends on the loads, basically.
If you have 30 pax on a 737 and they're all seated at the back for trim and balance, you do not need anyone at the overwings. The reasoning behind this is that it's more important to keep the aircraft in trim than the minute chance that you'll need to use the overwing exits.
If you have fairly high loads, then you need an able bodied person at each over wing exit.
That said, if you're on a flight and seats are blocked off for trim, you can sit in the empty seats during the cruise. The blocking off of seats is for take off and landing.
Hope this clarifies things somewhat.
Gg