I’d argue that your energy (or budget) is better-spent on a lot of other tactics than rank-tracking in local, given the importance of location and personalization.
Personalization (a part of a group of factors that relate to engagement) is playing a much bigger role, and Google’s now highlighting this explicitly in its products. If you or your clients still insist on tracking, Whitespark’s product is the best I’ve demo’ed.
For several years now, rankings have been at best a directional indicator of SEO performance — across the entire SEO landscape, not just in Local. And in Local, a more powerful directional indicator of SEO performance now comes from Google My Business Insights in the form of discovery searches and clicks to call and driving directions by Zip code. Google provides this freely via the GMB dashboard for individual locations and they’re working to make roll-up reporting easier for enterprise brands. With a little creativity you should be able to extrapolate a directional indicator of organic rankings for discovery phrases by individual Zip.
There are now reports that more detail around keyword queries may be coming to GMB as well.