Is SSL (secure headers) necessary for the entire site?

[mytheory.]

The title asks the main question of this post.

I noticed when I enabled SSL in the BO it applied those to only pages related to the checkout process. At the time I felt that our customers would like to see the https:// header from start to finish on our site. It would provide them with an overall sense of security. So I used a url rewriting tool (IIRF) to 301 redirect all requests to the https: header. Recently, I've noticed that there have been some SSL errors on IE (don't really use the browser but customers might so it's important). I've found some solutions online that I plan to implement to rid my shop of these errors (init.php solution). So thats not the issue, at least of yet.

My issue/question is: I read somewhere that if you just change the header requests to read https: that it may cause some problems with google's rankings. SEO being an important aspect but also security and that obligation to our customers... is it necessary to include the https: header to every page on the shop? Your thoughts...

Essentially, I would like to keep these secure headers throughout the site, but not at the cost of losing search engine rankings. Is there a way to achieve the best of both worlds? Or do you think that just keeping the check out process secure (pages) will suffice?

Any opinions and insight are appeciated. Thanks.