Sony makes a viewer for PC, so does Nook, so if you think you might go either of those directions, you could try those as well to preview. You will find if you begin to fiddle with embedded fonts, there are quirks there as well. They are not shown in Sigil, but appear in the viewers and devices.
Another reason to get and use a viewer is that images can overlap if they are too large, flooding the following page. This too is not shown by Sigil, but is shown by a viewer or device.