It isnt, we do it all the time and on every fleet that we operate! We like planning flights with a 1.3G buffet margin. In some cases where we have a "cheap" FMC, it will decide to send the aircraft to an optimum altitude of FL410 on almost every sector based on a 1.1g buffet margin, so by increasing the buffet to 1.3g, we reduce the maximum altitude.
Mutt