Roof bar spacings do vary. If you have fixing points it will be very obvious where you need to position your roof bars, but not for other roof types.
Roof rails (raised and flush) and T-track slots: for most roof rails you can position the bars anywhere on the rails. Positioning the bars approximately 70cm apart is considered ideal for roof boxes and bike carriers. You'll want to fit the bars far enough forwards so you can open your tailgate with the roof box (or other load) fitted.
Some roof rails have integrated fixing points, but your bars won't necessarily use these fixing points. Often it is just as secure for the bar to clamp around the rail, allowing you to fit the bars in the ideal position for your load (i.e. far enough forwards to ensure your roof box does not clash with your tailgate). Check the instructions tab to see exactly how the bars fit.
Plain roof: bars attach to a car with a plain roof using very specific clamps that secure around the door frame. The bars usually fit around 70cm apart but it's very important to check the instructions to see exactly where the bars fit. Most of our roof bars have an 'instructions' tab where you can check this. These instructions are usually supplied with the fitting kit.