I can give a couple of examples:
In the table that defines the categories and sub-categories for a navigation bar some links have subcategories (like accessories; narrow down the search by manufacturer, product model, etc) and some have none (like products; if you click latest products it shows the latest products, no subcategories).
Another example is I intend to include accessories and products in the same table. An accessory only requires a couple of images at most. A product will feature at least 4 images. There will be a field for image names... for accessories some cells will be blank. Furthermore, I might have 3 fields for product content (text) whereas with accessories it requires less text to explain what it can do (probably use 1 of the 3 content fields).
I am most interested in understanding the problems with leaving blank cells so I can consider whether I should bother to avoid it!?