You only have to replace them if the insulation is cracked. If this is something you want to do you can, make them cool colors or just get out of the house for a few hrs. As for lengths, well designed cars have all the wire lengths the same so that resistance and travel time is exactly the same. Yes it means coiling up the wire for the near plugs. Not a requirement but just letting you know.
If you are replacing the wires might as well put some good spark plugs in there.
If you are replacing the wires might as well put some good spark plugs in there.