If you are using <ul> and <li> elements for your links it's one fewer tag in your HTML:

    <ul class="nav">
        <li></li>
    </ul>

vs

    <nav>
        <ul>
            <li></li>
        </ul>
    </nav>

The only 'danger' is that if someone is specifically looking at your page specifically for <nav> tags to locate your navigation they won't find it.