I know this is an Allegiant thread but I think the following observation might apply to all the ULCCs.
At LGA the other day, while joining the TSA queue for a Spirit flight, I was briefly assessed by the private security/screening individuals checking the boarding passes. I normally carry a backpack on wheels that slightly exceeds personal item size. One screener asked the other, "Is this carry-on?", the other looked and shrugged so I was waved through.
I was once stopped at the gate for a Frontier flight for the same bag. I began an elaborate restructuring of items in the bag (it wouldn't have worked anyway) and because they wanted the flight to depart on time, again I was waved through.
I concur with the poster who stated to keep the bag away from the direct observation of screeners or gate/ticket agents.