No, the base URL does not matter. If you use dynamic content in .aspx or .html, anyway. If you use ASP.NET content with .html because of the requirements (however silly it may be), I suggest finding an alternative extension (e.g. .htm) for all static content. You do not want your static HTML files to be processed unnecessarily.
As Femaref said, you can use sitemaps to help.
Also, make sure your URL does not change (including variables) if the content is the same. This should not be a problem with MVC.
Edit: In your example: mysite.com/items/getitem/5.html
I assume you originally wanted: mysite.com/items/getitem/5
No extension matters. Since this is not a problem, I would also argue that the extension makes the URL less "clean", and also assumes that there is a file named 5.html in this path, which is obviously not true.
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