Illustrator CC has a couple of new features for creating ruler guides:
1. Double-click on a horizontal or vertical ruler to create a guide at that location. Add the shift key when you double-click to snap the guide to the closest tick mark on the ruler.
2. To create both a horizontal and a vertical guide at the same time, drag from the intersection of the horizontal and vertical rulers while holding down the command key (Mac) or ctrl key (Windows).
Bonus tip: (This isn’t new in CC, and I don’t recall how many versions back this one works.) Normally, ruler guides span the entire canvas, which can be a messy problem if you are using multiple artboards. To create a guide that is limited to the current artboard, choose the Artboard tool (shift-o) and drag the guide out of the rulers with that tool.