updating web.config by programmatically in application start up event of global.asax, is it right way to update the config file.? What impact it does on website.?

Question

protected void Application_Start(object sender, EventArgs e)
{
//In this event i m updating the web.config file...
XmlDocument doc = new XmlDocument();
doc.load("..../web.config");
//Logic to update the web.config file.
}