I guess it's a matter of preference, and I can see advantages and disadvantages to doing it either way.
Personally, I prefer the gplot way - only plot the markers that fall within the axes. Imho, if you plot markers that are outside the axis min/max values, then you could run into data-integrity problems of the end user assuming the markers fall within the axis min/max values, when they actually don't (this is especially problematic when the values are just barely outside the axis min/max).
That being said, I believe that is the way the new 'proc sgplot' handles axes, so you might want to give that a try, if you've got v9.2