What difference does it make whether I use true italics or not?
In an experiment with true italics and fake italics, in my page layout program I set some text in italics with and without the italic version installed. See the illustration above the sidebar for the results. I didn’t have Century Schoolbook Italic installed so the software simply slanted the Century Schoolbook font a little. After installing the true italic version, the software used it. See the difference in appearance? Notice the change in the shape of e and a and the bottom of l and i.
Not only does the use of the actual italic fonts make a difference in appearance, it can make a difference in what gets printed. When having a document commercially printed, if the italic version of the font is missing it is possible that the text would not show up at all as italic (fake or true italics) when printed.