I'm assuming you mean the single.php file that is in your theme folder? Remember when you update WordPress it only updates the root WordPress files and doesn't touch the files in your theme folder. So you should be fine. Someone please correct me if I'm wrong.
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Don't put the cart before the horse. Plan your website, thencreate it.
Lisa you are right for the most part as most of the security issues with the theme handling have been solved. I have updated several WP sites and there seems to be no changes to the theme index.php The only update I have seen that caused issues with themes was back a few years ago when they where still in the RC stage.
Your host will update their servers in time yes. but they will not update your blog. As your blog is in a separate folder from what they update. Basically they will only update the install files for their service. Once installed your on your own.