Well you would only be checking for the specific version of the .NET framework that your application truly requires, so that path/filename are constant.
Also, whether or not you you automatically have all of the previous versions, depends on what .NET frameworks were previously installed.
'cause, on a clean system, the 2.0 framework for example will also install 1.1 and 1.0. On a dirty system, on the other hand, where you've already installed an earlier version of the framework, then must install each one individually.
For our original discussion on all this, look
here.