You don't have to deploy your optimized XAP file to test it. One way you can test your optimized XAP files is by adding it to the build process and then running the project from Visual Studio. For more information, see Adding XapOptimizer to the Build Process.
You could also embed your XAP file in an HTML page that you can open and then refresh after optimization (see Adding Silverlight to a Web Page by Using HTML or JavaScript at MSDN for more information: http://msdn.microsoft.com/en-us/library/cc838217(VS.95).aspx) or reopen the TestPage.html file that Visual Studio typically creates when debugging or building a Silverlight application and that should have your XAP file already embedded (generally located in the ProjectName\ProjectName\Bin\Debug directory).