Adhering to Theme
Not every piece of your stationery suite will require postage, but most, if not all, of your paper wardrobe should conform to the same color scheme and unifying theme.
So, for example, if you're planning a Valentine's Day wedding, you don't have to include hearts and cupids on all of your stationery, but you could feature red and white prominently, use a unifying font and select the same material or grade of paper stock for all your stationery pieces.
These similarities will unite the suite and help it support your overall wedding theme. It's also easier to find a single stationery theme and stick with it than to come up with new ideas for all your pieces.