Depends on the type of bike this is. 700x23 is pretty narrow, usually suited to fast road bikes. If this bike has 700x28 tires, it sounds more like a touring road bike or hybrid. It also depends on your needs. Do you want to go to as narrow a tire as possible? If so, you could probably go down to a 700x25 and the bead will still catch the rim. But unless you want the reduced rolling resistance and added speed of that narrow a tire, I'd stick with one as close to the original as possible.