You might end up with this requirement quite often to set your custom page as a welcome page for site/subsite instead of default.aspx provided by SharePoint.

Its quite a tedious job to set up default pages manually when you have more than 10 site/subsites. Its pretty easy to do it programmatically. Here is how:

public static void SetDefaultPage(SPWeb web, string pageName)
{
    if (PublishingWeb.IsPublishingWeb(web))
    {
    PublishingWeb publishingWeb = PublishingWeb.GetPublishingWeb(web); 

    //Get the file name
    SPFile welcomeFile = web.GetFile(pageName);
    //Assign the new filename to the DefaultPage property
    publishingWeb.DefaultPage = welcomeFile; 

    //Update the Publishing Web.
    publishingWeb.Update();
    }
} 

Remember to pass the full relative path to the pageName. For example, pageName should be "/Subsite/Pages/MyNewWelcome.aspx" if your welcome page resides in a Page library of the subsite.