Thanks for all who gave me hints. I managed to solve my problem programatically by responding to event : ParameterRequestBeforeShow Then the report will be shown with today's date as default value for parameters and user is still allowed to select different date.
private void XtraReport1_ParametersRequestBeforeShow(object sender, DevExpress.XtraReports.Parameters.ParametersRequestEventArgs e)
{
this.Parameters["StartIssueDate"].Value = DateTime.Now;
this.Parameters["EndIssueDate"].Value = DateTime.Now;
}