Leave them as is. A very common option, but dirt & debris get stuck in there making cleaning difficult.
Fill them with a elastomeric polyurea compound that will stretch and contract as your concrete moves. This will also help protect your concrete at the joint lines. It eliminates a blunt impact by wheels and dragging objects. It also makes your floors far easier to clean. Not to mention safe for high heels, roller blades, pogo sticks, and the like not getting stuck and preventing a potential tripping hazard.