Can one cause Page.IsPostBack to be true independently of ASP.net?

If one is checking a user’s roles to determine whether they may access a page, is it safe to put this check only inside an if (!Page.IsPostBack) { ... }? Could it be possible for the client to cause Page.IsPostBack == true independently of ASP.net; that is, the client POST’s to the page and sets the right form fields? If that were possible, then I suppose best practice would be to check security on every page load, not just when Page.IsPostBack == false.