I would ask what the impact would be if you do not use the canonical tag?
Answer? Disastrous.
Since you have duplicated content, then you are effecting your search performance. There is a penalty in the SERPs for this. You will not know otherwise. It is likely that the penalty exists for you now and you may not realize it assuming that this condition has existed for a period. You may already be penalized. It is time to fix it. Fortunately, this is not only easy to fix, but should reconcile the penalty soon enough.
Use the canonical tag to point the original content on your company site. Given how Google clusters pages, domains, etc., your parent site should be the original. For all other pages, they will not rank for the content with one exception- locale.
Make sure that each of your reseller sites has proper schema.org mark-up for the location. As well, this is important for the parent company site. The reason is simple. While the mark-up is not required, it is more trusted in the eyes of Googles index. I recommend using as much detail in the mark-up as possible. Lack of detail can defeat the process. Physical address, phone number, and e-mail address should be enough. Names help of course. Using Google maps and other Google options such as ones related to businesses should be a consideration as well. Do as much as you can right away, and possibly more for the future.
What happens after that is Google will begin to figure out the relationships between the sites and that the reseller sites are location specific. This may have happened already. I am recommending schema.org mark-up to increase the odds that it happens or to begin the process. The reseller sites can then be used for locale search whereas your parent company site would perform better for non-locale searches.
It is not likely that a reseller page will out perform the original with the canonical tag in place except where it should. Google does get this right it seems. They have been at this specifically since 2007 when locale began to take a bigger presence in search intent. Since then, Google has had plenty of time to get it right.
As with anything search engine related, it does take time. Google will have to revisit each page and that depends completely on the freshness and importance of the page as Google sees it. If there are not too many, you can always use the Fetch as Google option a few a day. There is a limit of course and I do not suggest reaching that limit. Pick a fair number and submit only the pages with the mark-up starting with the parent company (no reason except that there may be a strategically important benefit). If the limit is 50, then perhaps submit 20. Try not to poke the bear too much.
You will find in the end that your site will perform more as it should. If there is a penalty applied, and again, you may never know, it can take as much as 6 months to regain proper traction in the SERPs. However, it is well worth the effort! It is far better to lack in performance for the little things and not the big things. Sage advice since this is how we are judged.
