AngularJS - Do I need to call $apply if no model is changed?

Greetings Overflowers,

If I am changing an HTML native attribute (say a division's class) inside an event handler, do I need to wrap it with a call to $apply? In other words, are these native attributes watchable?

EXAMPLE:
I am doing a custom directive inside which I am modifying the element's classes for styling on certain events like mouse enter, but no modification to the scope (or model). I am wondering if I need to surround this logic with a call to $apply just in case there is a $watch on these element's class attribute? Are these $watches possible using angularjs? My custom directive will be used by other programmers.

Kind regards