How does a constant pressure chart affect your preflight planning?
As others wrote, I don't think in and of itself the constant pressure chart will help with a go or no-go decision by itself.  However, I do believe it would help to get the big picture of what's happening with the weather; fronts, etc.  Knowing the big weather picture is more important depending on what kind of flying you do.  The constant pressure chart can help with that.  Regarding choosing a flight level or altitude to fly in I would use winds aloft, airspace charts like a sectional, anticipation of ATC given altitudes in the area you plan to fly in, etc.  Like many others have said: with regards to weather the more you know the better off your preflight planning will be and the constant pressure chart adds to your knowledge of the weather.