So you want to make sure that you always print in landscape mode even if the user doesn't choose landscape, but the PrintJob property "landscape" is read only, and there's no way to force the user to choose landscape. In addition, PrintJob has some calculation inconsistencies with rotated clips. There is a solution, and here it is.

Pass this function a movieclip and it will make sure it prints in landscape mode. It rotates the clip, handles all the measurements, and rotates it back when it's done printing. This all happens invisibly to the user.

function printImage(mc:MovieClip):Void
{
        var realW:Number = mc._width;
        var realH:Number = mc._height;
        var orgX:Number = mc._x;
        var orgY:Number = mc._y;
        var pj:PrintJob = new PrintJob();
        var pageCount:Number = 0;
        if (pj.start())
        {
                mc._x = 0;
                mc._y = 0;
                var cXscale:Number, cYscale:Number;
                if (pj.orientation.toLowerCase() != "landscape")
                {
                        mc._rotation = 90;
                        mc._x = mc._width;
                        cXscale = (pj.pageWidth / realH) * 100;
                        cYscale = (pj.pageHeight / realW) * 100;
                }
                else
                {
                        cXscale = (pj.pageWidth / realW) * 100;
                        cYscale = (pj.pageHeight / realH) * 100;
                }
                mc._xscale = mc._yscale = Math.min(cXscale, cYscale);
                if (pj.addPage(mc, {xMin:0, xMax:realW, yMin:0, yMax:realH})) pageCount++;
        }
        if (pageCount > 0) pj.send();
        mc._xscale = mc._yscale = 100;
        mc._rotation = 0;
        mc._x = orgX;
        mc._y = orgY;
        delete pj;
}