It is a personal preference. PADS has the long standing 'bug' of not supporting negative coordinates in the CAM output, so if you don't use the lower left corner you'll need to use offset in the CAM output. I got in the habit long ago of putting the lower left corner at 1000, 1000 so i could have dimensions on teh left/lower side without requiring an offset (although I typically use offset anyway for consistency between the CAM files). .
Occasionally a PCBA will need a common reference with another PCBA or mechanical fixture so it makes sense to put the origin at that point. it varies by design and designer