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 :)
Don't look at posture as a matter of right and wrong, correct or incorrect.  Ideal posture exists for the sole purpose of helping you develop concentration.  The general idea is that an upright body equals an upright mind.  Slouching is going to cause you to fall into sleepiness.  Your mind will go as slack as your body.     

On a more mundane level, if you tend to tilt forward in your sitting, a couple of other things are going to happen.  For one, after about a half an hour, the middle of your back is going to start to hurt.  You might also feel tension in your shoulders as your muscles struggle to keep your body upright.  You might also notice that as you lean forward, you start to restrict air flow.  It's not like this is going to cause you to pass out, but it will be harder to do deep, abdominal breathing.

And remember, this is an ideal.  No one is going to smack you upside the head if your posture goes a little slack (well, unless you are studying at a Rinzai temple!).  Maintaining correct posture is only going to benefit your sitting.