Too deep is relative to you and you can look at it this way: whenever your form begins to deteriorate at a certain point, then that is the "too deep" point to avoid. If you squat perfectly to parallel but for whatever reason lose form beyond that point, then working on form would take precedence over going a centimeter deeper than parallel until the issue is rectified.
I hope my answer made sense to you Kindred.
PS: The deeper you go, the more muscles get involved to bring you back up. As long as there are no quick descents and bouncing at the bottom; you'll be fine.
Fadi.