It is vital to know the limitations of your business and be meticulous with your budgeting and cost control. If you increase your Promoted Listings percentage just because other sellers are beating you, this will undoubtedly diminish your profits. There will always be a bigger fish, with a bigger budget to knock you out of the water.
So, tailor your Promoted Listings to your own business objectives and margins. Play to your strengths, leverage your assets and be competitive within reason. For some listings, increasing your percentage might do very little in terms of sales. A lower ad rate, or none at all, may be just right for those products.
It really depends on the product itself, how in demand it is, and whether or not the listing can be optimized in other areas. You should also have a good grasp of your item’s sell-through rate (conversion rate) prior to promoting the listing, so you can analyze how it performs over time.