I personally wouldn't worry about it too much, although if it did begin to crack it would result in the rim eventually splitting in 2 (but you'd most likely have a wheel that couldn't hold any pressure before that happened).
If you wanted to ensure it would last as long as possible, you need to blend out the scratch. Spin the rim 'round while holding a coarse scotch brite on the scratch, & work your way up to a fine scotch brite or some really fine wet & dry. If it's really smooth a polished it'll reduce the chance on any cracks propogating.
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