If you are using the simple toner transfer method you will need to mirror the image only for SMD components.
If you are using a resist pen, you will have to mirror the image because the components are on the other side of the board, this will make it easier than just working it out in your head.
If you are using the photosensitive boards it doesn't which way you print it because you can flip it over to suit the overlay.
If you are getting them made by a PCB manufacturer they will sort it out for you.
if you use toner transfer you should transfer the toner right onto the copper layer.
When using UV-sensitive PCB material you should mirror the component side only for direct contact of the print with the photo sensitive layer. The bottom layer will be printed for direct contact with the photo layer, since the image is a "look-through-the-PCB" image.
Even using transparencies of 0.1mm thickness stray light (reflections) will over expose fine traces.
I fully agree with you having made the experience that fine traces diminished while developing the PCB with the toner on the wrong side of the film for UV exposure.
Once again: Bottom layer: mirror? NO,
Top layer: mirror? YES