Is a straight spine/spinal posture during seated meditation critical?

I notice, in images of monks and lay people in seated meditation, that many of these various persons appear to be using very poor posture, e.g. slumped and not erect. Written instructions and video instructions often recommend adherence to a straight spine and/or an erect posture. I think the Buddha simply instructed that one sit, without specific instructions about an erect straight-spine posture. Therefore, I find myself confused and distracted regarding "correct" posture. My breathe seems to be more natural and relaxed if I "slump" just a bit and dispense with the "straight spine". Although, I've also noticed that when I notice that my attention to the breathe has wavered I have gravitated into a more pronounced "slump"! SO, what is most acceptable and correct? Thanks for any advice that is offered :)