Underlined values indicate that a value has been entered manually and not calculated by the Timeline. Whenever you change a value it will have an underline unless it is exactly the same value that the Timeline would have calculated.
When you initially create a Timeline, the Planned Start date for the first Task and all the Durations will be underlined to indicate that these values have not been calculated.
The underlines WILL print and there is no preference for turning off the underline feature.