This is fine but if we have too many user controls across the sites (and that too ascx files) then it can be painful to manage across the site.
The control declaration is much cleaner and easier to manage in Asp.Net 2.0. Instead of duplicating them on all your pages, just declare them once within the new pages->controls section with the web.config file of your application
The controls need to be added in the controls tag inside the pages tag which will be inside the system.web tag.
An important this to note here is to use the ~ path as the user control can be used anywhere in the site. The “~” will resolve the control from the root of the web site.
Once the control is registered in the web.config file the control can be used by any page or user control in the site.
Another Important thing to note is that there is no performance difference in either registering the controls in the web.config or on the top of the page as they get compiled down to the same instruction in both the scenarios.