Je sais que c'est il y a longtemps mais cela peut être utile à tout autre nouveau venu,
J'ai décidé de transmettre le nom d'utilisateur, le mot de passe et le domaine lors de la demande de rapports SSRS, j'ai donc créé une classe qui implémente IReportServerCredentials.
 public class ReportServerCredentials : IReportServerCredentials   
{
    #region  Class Members
        private string username;
        private string password;
        private string domain;
    #endregion
    #region Constructor
        public ReportServerCredentials()
        {}
        public ReportServerCredentials(string username)
        {
            this.Username = username;
        }
        public ReportServerCredentials(string username, string password)
        {
            this.Username = username;
            this.Password = password;
        }
        public ReportServerCredentials(string username, string password, string domain)
        {
            this.Username = username;
            this.Password = password;
            this.Domain = domain;
        }
    #endregion
    #region Properties
        public string Username
        {
            get { return this.username; }
            set { this.username = value; }
        }
        public string Password
        {
            get { return this.password; }
            set { this.password = value; }
        }
        public string Domain
        {
            get { return this.domain; }
            set { this.domain = value; }
        }
        public WindowsIdentity ImpersonationUser
        {
            get { return null; }
        }
        public ICredentials NetworkCredentials
        {
            get
            {
                return new NetworkCredential(Username, Password, Domain);
            }
        }
    #endregion
    bool IReportServerCredentials.GetFormsCredentials(out System.Net.Cookie authCookie, out string userName, out string password, out string authority)
    {
        authCookie = null;
        userName = password = authority = null;
        return false;
    }
}
en appelant les reprots SSRS, mettez le morceau de code suivant 
 ReportViewer rptViewer = new ReportViewer();
 string RptUserName = Convert.ToString(ConfigurationManager.AppSettings["SSRSReportUser"]);
        string RptUserPassword = Convert.ToString(ConfigurationManager.AppSettings["SSRSReportUserPassword"]);
        string RptUserDomain = Convert.ToString(ConfigurationManager.AppSettings["SSRSReportUserDomain"]);
        string SSRSReportURL = Convert.ToString(ConfigurationManager.AppSettings["SSRSReportURL"]);
        string SSRSReportFolder = Convert.ToString(ConfigurationManager.AppSettings["SSRSReportFolder"]);
        IReportServerCredentials reportCredentials = new ReportServerCredentials(RptUserName, RptUserPassword, RptUserDomain);
        rptViewer.ServerReport.ReportServerCredentials = reportCredentials;
        rptViewer.ServerReport.ReportServerUrl = new Uri(SSRSReportURL);
SSRSReportUser, SSRSReportUserPassword, SSRSReportUserDomain, SSRSReportFolder sont définis dans les fichiers web.config.