@JaredPar has the correct tricks to autocheckout a file on a post build event.
My question (warning?) to you is : why are you trying to OVERRIDE the source file with a minified/combined javascript file?
In my opinion, if your doing some post-build event to minify/combine some javascript files into a new destination file (Production.js) which is never touched .. it's just the output of post-build event ... i would suggest you don't do this in a post build event, but part of some proper source control deployment scenario.
For example, I've put up a video how to use TFS2008 with YUICompressor.NET which shows how you can make sure that your javascript are minified/combined whenever a checkin occurs.
Then again, you're using Visual Source Safe and that is a serious issue which I would address before -anything- else.