the generated thumbs from jpeg files (maybe other formats) that have exif informations should use the orientation value
Are we talking about Thunar or Tumbler here? Tumbler does not implement a fast JPEG thumbnailer yet. Instead it uses GdkPixbuf to load and thumbnail JPEGs. This will soon change. But are you talking about Thunar from Git master?
All I'm saying is that the thumbnail should be created with the right orientation. I don't currently know the result of the generated thumbs, but it's a common lack of the thumbs so it's just a prevention bug.