Yes, you can underline/highlight using the tablet. And while this may be useful to bring some ideas to life visually, such as underlining key tone words and main conclusions, I have heard feedback from many of my students that the highlight feature using the tablet and stylus provided is quite difficult to use. On this note, I would caution against an over reliance on this feature. However, do as you feel is right as it is purely a matter of preference!
I hope this helps. Please let me know if you have any other questions!