Might be better to use a template part I use these lots of times for all sorts of template add-ins, social media, menu's, footer text etc:
How they work, lets use your example of a navigation menu, I add two additional menus, top of page and footer.
I create two file, nav-1.php and nav-2.php, add my code in these files.
Then to call these files is as easy as:
<?php get_template_part('nav','1'); ?>
<?php get_template_part('nav','2'); ?>
The names can be anything, I use social-1.php and social-2.php for horizontal and vertical icon bars, logo-1.php for a logo etc:
Another use is sidebars for different template pages, lets say you have a template for members and you wanted a special sidebar, same process, new file sidebar-members.php, add the code to call the different sidebar, then in the template page replace, get_sidebar() with:
<?php get_template_part('sidebar','members'); ?>
If the sidebar-members.php could not be found then WordPress will load sidebar.php.
Example Twenty Ten theme:
This is how it deals with loops, if you wanted a different loop for the archive page with thumbnails and excerpts, you would save loop.php as loop-archive.php and make the code changes, and this would be used for archive.php, with loop.php as a fallback.
If I am right the include is ok for non child themes, but an include in the child will look in the parent directory, not 100% sure on that though.
Benefit is there will be no error if the file does not exist, the file could be in the parent or child theme and WordPress will find it, and once you use them you will find they are portable to new themes, drop in the 'template part' file and one line of code!
HTH
David