As long as all the controls used in your application support AutoScale you don't need to wory about it in your code. It is a way of running code written for QVGA screens on VGA screens without changing anything. The only thing you will see different on a VGA screen is that graphics quality is somewhat diminished and graphics text might rendered a slightly different width.
Only control authors need to worry about AutoScale. All the intrinsic controls in Basic4ppc version 6.76 are AutoScale aware and all my libraries, where necessary, will be reissued as AutoScale aware when the Basic4ppc version 6.80 is officially released.
What is with old libraries with custom controls ... Will they work with AutoScale if I just call them with all position and size parameters multiplied by scaleX and scaleY?
They will need updating to be recognised as AutoScale aware controls. This is necessary in order to have their Left, Top Width and Height properties automatically adjusted and they may need some internal code changes as well. You can use them without them being updated but in an AutoScaled application you will need to resize and reposition them according to ScreenScaleX/Y as they won't be resized automatically. Other code involving them may need to changed depending upon the individual control.
Another reason you might need ScreenScaleX/Y is if you are writing your application to be both QVGA and VGA aware and want to do the screen resolution compensations yourself and are
not going to AutoScale compile it.