traditionally 68/20 was the normal processing temperature as it was easy to maintain.
However the killer was sharp changes in temperature between baths and washes that could lead to reticulation and even frilling. ( emulsions are tougher today)
the temperature you can easily maintain your wash water is usually the way to go, as you can use it as a water bath to maintain all other temperatures.
The problem then is only if you can not maintain it with in about 8 degrees of 68. when other strategies are needed. like warming or cooling. (to avoid excessively long or short times)
The available time temperature curves and tables are very accurate and can be relied upon.