Current best practice in CSS development is to create more general selectors with modifiers that can be applied as widely as possible throughout the web site. I would try to avoid defining separate styles for individual page elements.
If the purpose of the CSS class on the <form/> element is to control the style of elements within the form, you could add the class attribute the existing <fieldset/> element which encapsulates any form by default in web pages generated by ASP.NET MVC. A CSS class on the form is rarely necessary.