I did an experiment with the image quality parameter of the imagejpeg() function when creating jpegs. I found the optimal image quality with file size is taken into account to be 80 - very close to the default value of 75.
Anything over 80 results in an unnecessary increase in file size without much increase in image quality.
Results and sample pictures: http://www.ebrueggeman.com/article_php_image_optimization.php