As far as offering to check bags for free, this would only happen if you go to the Ticketing Counter to get your boarding pass or check bags that are too large for carry-on. It is only your carry-on size bags that you did not intend to check that would be eligible for free checking. You still have to pay for the larger bags.
Usually on other airlines' regional jets, at the gate or on the boarding chute there is an agent doing some gate checking because RJ's smaller bins will not accept some carryons even though of legal dimensions, or the flight is so full it's obvious the bins won't hold it all. Allegiant didn't do any gate checking. Pax carried on a few bags that wouldn't fit and had to be offloaded after everybody was seated. The point is, don't count on the gate checking option with Allegiant.