Eh, they should all be as safe as any downtown would be. Not a lot of walking traffic likely next to the Jefferson or Monaco, as a lot of that is business sort of area that's mostly empty at night/weekend. The George might be marginally less "safe" given its location near Union Station, but I don't think it's truly worth worrying about. I've walked around these areas of NW DC at night without worrying.

If you're flying into DCA (Reagan National) and planning to use the Metro, make sure you check on the last run times. If your flight is a little late at all, you might be looking at a cab instead. Metro shuts down around 12:30-1am as I recall.