When deciding where to focus your SEO time, should you spend time on product pages that have Rich Snippets already or ones that don't have any yet?
Ideally both. There's good reasons to promote all of your products.
When you focus your SEO work on the products with Rich Snippets, you'll help them rank better which will lead to more traffic and sales.
On the other hand, by focusing your SEO work on products that don't have Rich Snippets yet (but do have the structured data for them), that might influence Google to give you Rich Snippets faster. It appears that Google analyzes the popular and most linked to pages first for Rich Snippets, so promoting a page could speed up it's Rich Snippet process.
If I had to choose, I'd focus on neither.
Instead, focus on the pages that are your most popular.
That's because your popular pages are already attracting traffic and sales. By boosting them, you're more likely to get more sales for the same level of effort you put in.
Continue that at least until you start to get diminishing returns and decide to focus on other pages.
The Rich Snippets will take care of themselves.
(The same advice applies to many other optimizations: conversion rate optimizations, page speed and performance, etc)
Of course, in order to get Rich Snippets at all you will first need to have a complete set of structured data for your pages.
JSON-LD for SEO can help you with that. It's fully automated process will add the structured data your store needs in about five minutes.
You don't even need to edit your theme yourself to do it.
Eric Davis