It depends on the airline, the number of crew, and the laws under which they operate.
On the 737-800 i've worked with a company that requires 2 in each block of 3 as they operate with minimum crew of 4 and thats what their 'exemption requires (its 1:36 ratio here still, however on the 738 they have an exemption).
At another company, that operated with minimum crew of 5 on the 738 (no exemption except in disruption/or unusual circumstances), this was not the case.
On the Embraer at the first airline, the exit rows need to be fully occupied from memory, as do the 767 exit rows at the second airline.
On the other hand, i've operated 2 (with exemption) and 3 crew ops on the F100 with no legal restriction (that was a few years ago).
As for charging pax - they're not really paying for much, beyond the guarantee that if they are assessed as able bodied and appropriate, they will have first priority for those seats. That said, more often than not, they are seated with others lucky enough to score the exit row without paying (usually frequent fliers or people who charm the check in agent).
So - it varies, from aircraft to aircraft, airline to airline, and jurisdiction to jurisdiction.