I was asked recently: ‘Our form has many required fields and few optional fields. We think that we should use an asterisk to indicate the optional fields. Do you agree?’
It makes sense but it is also a bad idea.
The problem is that most users spend most of their time on websites other than yours, and on forms other than yours. Their experience on other websites habituates them to the idea that an asterisk means a field is required.
If you then change that convention and try to make an asterisk mean that a field is not required then you are asking your users to learn a new convention purely to interact with your site.
I can envisage circumstances where this might be acceptable (for example, a small-population intranet where users are highly trained in that particular site), but my strong recommendation for all standard circumstances is that you should use the asterisk to indicate required fields rather than ask your users to learn the opposite convention for the relatively short time they spend with your site.