Should we be promoting all of our listings?

Should we be promoting all of our listings?

Yes. There is no reason not too- you only pay the percentage when you make a sale.
Also, if you aren't getting many views/watchers, after promoting the listing, optimizing titles, adding more keywords, adding a template...then you can also consider raising the promoted listing percentage. 

UPDATE: As of May 2019,  eBay now saves your promoted listings settings so that when you relist an item that previously had promoted listing (or use ebay's "sell similar" feature), it will save you time in applying the settings to the new listing.