Hold Shift to Snap Guides to Rulers.
This week's creative tip is one of those handy shortcuts you can use all the time. As basic as it is, a lot of us missed this feature along the way.
In many Adobe application (InDesign, Illustrator, Photoshop), hold down the Shift key while dragging a guide to snap to the nearest ruler mark. The cool part is that your guides snap to the visible tick marks on the ruler whatever the magnification level. Once you've tried it, you'll be using this trick every day.
Source: This tip came up during a Q&A session at one of Jason Hoppe's recent weekly mini-workshops.