Should I use a wire or soft sponge to clean a solder tip?
I use a semi-soft wet sponge along with the brass wool; the brass wool is good for  removing excesses of solder from the iron's tip, while the wet sponge is better at getting oxidized crud off.  As an aside, you need to be very careful when sanding tips; many cheap tips are plated and aggressive sanding will destroy that plating, rendering the tip worthless.