When you go to View >> Show >> Grid and Guides, you can adjust the spacing between the grid lines.

Also useful: when you have View >> Guides checked, you can add guidelines on different levels by copying and dragging the 0-lines (with Control-Drag) to the positions you want

- View >> Slide Master >> main master (the one with the number in the front) >> add guidelines here and you will see them on every slide, without the risk of dragging the guides accidentally; you can change the red color by right-clicking the guideline

- View >> Slide Master >> 'child' layout >> add guidelines here and you will see them on every slide you appoint that layout to, without the risk of dragging the guides accidentally; you can change the orange color by right-clicking the guideline

- Normal View: add guidelines here and you will see them on every slide; dragging them on one slide will drag them on the other slides as well; these are not 'fixed' so easily dragged by mistake - you can change the grey color by right-clicking the guideline