I can assure that we are actively assessing this feature and prioritizing it accordingly. Unfortunately I can't share the specifics if our roadmap as it is always subject to change as you have seen. We weigh many factors in deciding what to build next, the forums being one of them, so please keep the feedback coming.
Currently in a restaurant helping with a new POS set up and this honestly seems like a no-brainer :). Tips came up during their meeting so I had to double check for options... so yeah, extremely important as restaurant workers rely on tips and having a custom sale amount doesn't work as far as customer UX goes.
Item modifiers and tips are a must. Solve these two features (using Square, Breadcrumb, whatever as examples) and Shopify would become my defacto recommendation for web/POS integration and would probably be one of the first to provide a great end-to-end experience.
Pri 0! :)