We could also let wheelbarrows (and minecarts) be associated with stockpiles like bins and barrels are. They would then only be used for jobs relevant to that stockpile (or possibly workshop). Useful for furnaces, that need a lot of coal, carpenters that need wood etc.
How do they get back to the stockpile?
Finishing the hauling job would include bringing back the wheelbarrow. Or rather, the wheelbarrows would be at the furnace, and the haulers would got there to get instructions and equipment, do their jobs and when they're done a wheelbarrow full of coal is at the furnace, and the hauler free to take another job.
So an extra trip... (not that that's horrible, it just needs to be explicit. You don't start saving time until you're bringing three things at a time.
Bonus for wheelbarrow as a Labour setting: Jobs pick dwarves, so if wheelbarrow is a labour, the job to haul stuff in a wheelbarrow can be created ahead of time. The job knows it's a group job, the dwarf doesn't have to check. That means they are bundled from the get go.