Word 2010 has a wonderful built-in feature that helps you determine whether your file is safe to share. Open the document you want to send to someone, go to the "File" tab and click on "Info". There, click on the "Check for Issues" button and choose "Inspect Document".
This will open up a new window that asks you what things you want it to look for to see if you have in your document.
I typically leave everything checked, though I will often ignore findings in the headers and footers because I often intentionally put things there. Once you have everything checked that you want to inspect for, click on the inspect button, which will bring up a new window showing what was found.
Here, you can see that the document I had opened contained both comments and tracked changes as well as metadata (a whole post can be dedicated to this). To get rid of those things, simply click on the "Remove All" buttons to the right of the category. Once done, all of those tracked changes and comments will be removed from your document and it will be safe to send out.
Perhaps I'm overly anal retentive, but I do this on every single word file I email to someone outside my office. And by every, I mean every. And even then, I will often try to send a .pdf document instead if I can to avoid the potential problems associated with track changes and comments.
Again, I'm not saying not to use track changes or comments. I'm only saying, if you're going to, just be safe about it.