What nobody's mentioned is that, for some minor wounds at least, if you have no unassigned beds, the dwarf won't rest. I had a dwarf with a red arm for two years who went about her duties as normal, perfectly ecstatic. Her stats weren't through the roof either. She was a pump operator for a while, then I moved her up to clerk. It never changed from red, but she just held the pen/chisel/warhammer in the other hand.
When you're placing beds, make one extra, for the wounded. Place it and watch him/her bolt towards it. (Or recruit them and then place and lock them in a room I suppose.) Assign all the other beds to the dwarves in your fortress, and then remove any extra, including the one the wounded is lying in. As long as they're not passing out from pain or whatever, they'll be fine.
You've gotta watch out for if another dwarf dies though, as the wounded will cancel whatever they're doing and head for the bed the moment it becomes free. This trick is useful if you have a fortress with no water though, since they'll drink booze on their own quite happily. If they're already in a bed resting, then you can q > x it and it'll be taken away from under them. The wounded also makes a good indicator for unassigned rooms, like a dwarven dowsing rod, just open the door and let it go.