I do a lot of local SEO and some national/traditional ecommerce SEO and In my experience, you are absolutely limiting your "national" reach by having geo-tags in your titles. While no one can say with 1000% absolute certainty that Google is devaluing your 'National' search engine results due to your local geo tags, you're at the very least reducing your KW density for your page titles, which undoubtedly will affect your rankings nationally.
What I have found to be a solid strategy for eCommerce sites seeking local and 'national' customers, is to build landing pages that are city based subdomains that are optimized for a particular city. Then have links within those geo landing pages to the main ecommerce site.
example:
Tulsa.Widgets.com
Page Title: Tulsa Widgets | BarnDoors.com
Notice I'll usually keep the root domain in the geo subdomain for branding purposes and to avoid consumer confusion when they are looking at the SERPs. The good part for these local subdomains, is you can typically achieve quick rankings in these mid-size and sometimes larger markets. Most importantly though, this frees up your root domain to try to rank for your highest trafficked keyword at a higher kw density.
This strategy has worked well for my clients who are doing the brick and mortar + eCommerce thing but feel free to shoot holes in it - I'm always looking for better ways to handle the whole local & national seo issue.
Lastly, I would also add the caveat that if you're already ranking 'nationally' with your geo infused titles, (for example, if "Tulsa OK Widget | WIdgets.com" is ranking for the keyword "Widget") I wouldn't change a thing until there are signs that you're slipping.