What do you guys do with your scratch files? I'm considering deleting scratch files from weddings that I've already delivered, especially since I have an archive of MiniDvs. Thoughts?

What's a "scratch" file? You mean the NLE project/raw footage files used to make the final video?

I wipe them out whenever I need the drive space. After your contractual obligations are filled, why do you need to save them?

I backup all the project files and assets created for that particular job.
In my case this translates into .pproj files, .ncor and several .ai and .psd. As you can see I work with adobe (cs3).
If the job has a SDE clip or a short-form clip I backup it also in .avi format for quick reference/adjustments later (i I want to use them in portfolio or whatever).

In your particular case (seeing your website) it would be wise to do this: if a customer comes a year or two from now asking for a real HD version all you have to do is re-import HD versions of the footage to your project and export to HD-DVD or Blu-ray.

I'm starting to tell customers they can have an HD version in a year or two when it comes available to all. It seems to impress them and may guarantee new jobs by the time they ask for it. Also, by doing this I make a point that we are not a "production-line" type of business: we take pride in standing by our work for years to come.

I delete all of the scratch files and just keep the EDL & Master Tapes in a secure spot (fire-proof storage safe). If there were pictures/audio I also burn these to CD and file that as well. I export all of the edited pieces (ceremony, highlights, dvd menu) back to tape as HDV. I am not offering any kind of HD package, but keep it in HD for future demo use.