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