I have to agree with earlier posters that the airline is very strict with carry-on...Overhead bins are EMPTY, and there should be little concern for safety since many bags can and do fit underneath a seat. Bags can become heavy with books, paperwork, electronic equipment, and computers (although the airline is more lenient with the latter). Anyway, agents do have some leniency as to how many kilos over they charge you (they will often reduce it if you show a concerted effort to be nice and follow their requests), but you will still have to pay.
At least a dozen pax showed up at the gate with more than 7 kilos (very easy if you have a few books, camera, and laptop), and they were hilariously asked to unpack in front of everyone (where they stuffed items into their pockets and then put them back into carryons before boarding). As much as people grumble, 7kgs is too small of a carry-on restriction.
Air Asia's fares are low enough that this type of greedy upselling is tolerable, but budget travelers should beware.