The full theme running right through the credits is rarely if ever used any more. Although I think when they have special end credits like when Stacey left on the plane and we had the credits over the shot of night time london would have suited the full Julias Theme perfectly.
On another note, the full Julia's Theme must be one of the most beautiful pieces of music ever composed. Its so lovely and suits both sad and happy endings. I could listen to it all day.
Didn't the credits used to continue in the style of Julia's Theme?
Not as far as I can remember. It's just that they use a slightly different arrangement now which doesn't flow into the main theme as well. Here's the first episode it was used in: http://youtu.be/Xsjh6sqmju8?t=27m40s