What is the best method for editing/managing metadata of digital photos?

You can also manage all of these tasks in Lightroom. As the images are imported, you can have all of the Metadata applied. Lightroom is also useful for a ton of other tasks.

As a Windows user, probably your best bet is to use Picasa. It is simple and does everything you’ve mentioned plus much more. However, whether or not your metadata is preserved across platforms and web sites is entirely dependent upon them. There are standards for metadata, but if you upload a photo to a site that doesn’t use that information there isn’t much else you can do.

Both Lightroom and Aperture are excellent.  Lightroom makes life quite easy because you can also save a set of metadata to use for other batches you bring in; you won't need to re-type anything.  Something else to think about is to save those scans as DNG, or digital negative, which is cross platform and not tied to anything proprietary.  Good luck!

The information that you are looking to add is not really EXIF, it's IPTC.  Programs like Lightroom (Aperture is Mac only) will do it, but it's a bit pricey if you only want to add information to scans. Irfanview can add and view IPTC information.  It's a great, free program. There really isn't a way to ensure that the data remains in the file.  The file format isn't designed for data security, and many programs are not metadata aware and will strip out the data if you save the file in that program.  The best way to ensure the data is to keep a master copy with the data intact.

Use software intended to help with this. I use Aperture (for Macs). You import the photos and then work with keywords (its term for metadata tags). You can also edit other metadata fields and create custom fields. Besides providing a handy interface, the main metadata benefits for Aperture are 1) You can set metadata for multiple photos at once, and 2) face recognition. You give it the names of the faces it recognizes, and it will tag and organize all the photos with those faces (sometimes it needs some help, but overall it's pretty good).

As a Windows user I find Windows Live Photo Gallery meets my needs. It will identify faces, allow you to add location info (country, state, city but not lat/long), star ratings, captions and tags (equivalent of PhotoShop Elements albums). There's a also a simple boolean flag you can use for workflow status. You can scan directly into Windows Live Photo Gallery if you wish. All of the metadata you add is maintained in the EXIF data. You can bulk tag photos by selecting multiple photos with the usual Windows selection mechanism and assigning a tag, location, date, etc.   The filtering of views by date, image type (includng video), star rating, tags, folder and person is fast and efficient. It can publish to Flickr, FaceBook, YouTube, SkyDrive. I have been using it for years and find it does everyting I need for day-to-day work. I have a combination of scanned images, photos from half-a-dozen cameras, plus machine-generated images and hours of video.