Soldering wires in a particularly old harness will actually reduce resistance in that portion of the wire. It won't be noticeable except for allowing for greater signal/line integrity. I.e. In a wire with failing spots (where conductor has been damaged or has broken) or exposed conductors, rejoining via soldering will restore the reliability of that particular wire.
Soldering broken wires, extending stretched wires to reduce stress, etc. are very common processes. When done correctly, you will not affect the performance of the conductor. What actually will affect your harness more is the age and growing parasitic resistance of the harness itself.
Ideally however, best practice dictates that whenever replacing the harness, it's best to replace it connector to connector instead of splicing wherever possible. This is because for every solder joint, you theoretically introduce a possible failure point. Is that of major concern? Not particularly if you are skilled enough to produce good solder joints without cold jointing, insufficient wetting, and the numerous problems that can plague a solder joint. This will affect reliability, as well as introduce interesting anomalies.
In summation, provided you can terminate solder joints effectively, and protect them adequately (please don't use electrical tape to cover your solder joints - heatshrink is certainly a worthy investment for the longevity of your harness to prevent moisture and corrosion from penetrating the insulation) you should have absolutely no issues. Ideally you should just replace the harness from one major connector to another if possible, however in some cases this is not possible, and soldering wires is the only remaining option. This isn't to say soldering can't be as effective or durable, there are just many variables that can make it particularly difficult for novices or the inexperienced to maintain quality, repeatable, and effective solder joints.