i found a post from smokey in 2004 - seems to know what he's on about
http://www.pprune.org/archive/index.php/t-131271.html
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Jet aircraft have been routinely flying above Mcrit long before the super critical wing came along.
A large exceedance of Mcrit will yield a host of aerodynamic / control problems, including the onset of 1G 'Mach buffet'. The manufacturer obligingly places Mmo just below the threshold of aerodynamic problems, leaving a portion of the flight envelope between Mcrit and Mmo for normal operations - The problem then becomes one of additional drag due to the shock waves - wave drag.
Wave drag increases very slowly at first, up to about M0.05 above Mcrit, and then rises exponentially. The new drag curves (High speed polars) attributable to wave drag can be superimposed over the total drag curve (Low speed polars) and show some interesting properties.
Maximum Range Cruise (Cost Index zero for all you FMC users) is found by at the point where a straight line originating at 0/0 is tangential to the 'revised' drag curve. Due to the very shallow curve of the High Speed Polars for the first .04 or .05 above Mcrit, this 'Maximum Range Plot' invariable leads to Maximum Range Cruise just above Mcrit.
Consider Long Range Cruise, which by definition offers 99% of the economies of Maximum Range Cruise or 1% additional fuel burn per mile. The line is drawn from 0/0 at a 1% steeper gradient than that for the Maximum Range case, and has a low speed and a high speed intersection with the Total Drag curve. The higher plot (Long range Cruise) is at a point even further beyond Mcrit.
Econ cruise, where the total cost of airframe time (best at high speed) and fuel cost (best at Maximum Range Cruise) is at a minimum. In byegone times, when lower fuel costs prevailed, higher fuel useage was tolerable against relatively expensive airframe time. In the 'fuel expensive' modern era, Econ cruise usually falls between MRC and LRC.
All 3 of these cases indicate that normal cruise operations, even when aiming for maximum possible range, occurred at speeds above Mcrit. The only exception would be low altitude operations when Mcrit is not reached, but this is not the usual habitat for the jet aircraft.
The discussion above spoke of tolerable levels of drag in the cruise phase. Even small drag increases introduce penaltys in climb performance, and here, flight above Mcrit is avoided, or minimally tolerated. If you don't know thr 'generic' Mcrit for your aircraft, the climbing Mach Number will be a pretty good indicator.
Descent employs much the same tolerance of small amounts of Wave Drag as does cruise. Indeed, the rapid onset of drag for a small mach number increase above normal cruise speed will be most beneficial (and demonstrable in the simulator) when maximum possible rate of descent is desired during an emergency descent.
I hope that this helps to fill any remaining gaps in your understanding."