Advertisers prefer right pages. Since advertisers want great exposure that’s why they insist to be placed on the right page. Again, as you skim through the magazine you will notice their ad much easier. Especially if the ad is in vertical half a page format. Placing that ad in the inner part of the page, near the gutter, would be great mistake. Costly mistake.
ALWAYS LOOK AT A SPREAD AS A UNIT
Left pages are great for editorial content. It is always good to know which ad will go on the opposing page. In this way you can design editorial page in a way that will correspond with the ad. It is best to make a contrasting design on your editorial page. For example, if the ad is in blue shades, you should not use blue as central color on your page. If the ad has emphasized image that bleeds out, you should design your page with more text and very few images. In this way reader will have no problem do distinguish what is editorial and what is ad. Of course sometimes this is hard to carry out but try to make your editorial pages different from ad pages.
Try to remember these rules and follow them, once you are familiar with them you can start to break them, but only if it will bring something interesting and make design better. Do not do something different just for the sake of making it different. Always have meaning for whatever you do.