I think it depends on how/where they're being used. Something that lands on a pad might just have small landing feet, either retractable or fixed. If it's a street car there's no real reason for them to be retractable, and it'd just be one more thing that could break at the wrong time. Or, if air cars are supposed to be as widely used as ground cars, forgotten by a tired/sloppy/drunk pilot. Lockable wheels so it can be moved while powered down are a good idea IMO (a fair number of helicopters have these).
A vehicle that's supposed to land on undeveloped ground will probably have larger landing gear to decrease ground pressure. How large depends on the terrain it's meant for: snow, water, and deep sand need pontoons, soil or hard ground just skids or wheels.
Basically IMO helicopter conventions apply. Maybe with more emphasis on retractability for military vehicles that merely hover or "skim" instead of flying proper, for the sake of ground clearance on rough terrain.