Sketching with ink is definitely the right strategy for the UX field. Ink is so rigid and yet so flexible.
Some people think about ink as a tool that locks you in, but really I think it unlocks you. Other tools lock you into the viewscreen (or sheet of paper). You think, how can I manipulate what is on this page to work, and you use other tools such as an eraser to correct mistakes and manipulate based on change, wasting valuable time.
Take this simple sketch I did of a small profile pic for a social site. My initial design shows the pic and the overlaid text and some options.
Then, I think, this shouldn’t have overlay text or additional tools. Bam, done! A little bit of chicken scratch later and we are on the road again.
The key here is to not get too attached to a perfect image, which may never be attainable. Also, keep a fat stack of paper with you so you can just toss it aside and continue sketching.