since, after all, you *do* meet Garrus at the bar.....
I finally did Citadel on my Garrus-romancing infiltrator recently, and man was that dance number a hoot. Probably the best date cutscene I've seen so far. Good thing I made a hard save right before it so I can revisit it whenever I want.
As for Leviathan, I'd say it's probably best to always wait til after Rannoch, since you have all of your squad by that point, and even if you don't use all of them on the missions, they at least have unique post-mission dialogue back on the Normandy.