It appears that embedded images are scaled down to
660 × x pixels for display - does this apply to all screen resolutions?
I'm considering resizing my images when quality is not a major factor.
The images themselves are not resized in the posts, however the width of the post container itself (which contains the whole post) is fixed, and set to 660 pixels, in a CSS rule.
The images have width of 100%, so their height is auto resized by the browser itself to match the width in order to preserve the ratio.
So yes, you can safely resize your images to 660px width and it won't affect the way they look in the post itself.