You'll need to export once for each Styler stylesheet. But remember that Styler stylesheets can cover multiple output formats within a single stylesheet. For example, you can have one stylesheet that knows it should make titles blue for web output, but black for PDF output, etc.
If you have actual separate Styler stylesheets for each output, then yes, you'll have to export an XLIFF file for each one. But if you have one stylesheet configured to produce all your outputs, then you only need to export once.