instinctively I'd not bother with a decoupling cap on an op amp, maybe a power amp (not read the article yet as I should get on with my decorating).
I've seen stuff run fine without a decoupling cap as well, hec the crap water pumps with a pic based water detecting circuitry that they bought at work (long thread about it some where) managed to work with with no cap at all from the 78L05 reg to the pic but the power line was pretty ropy, there were -200mV and +300mV spikes on the 5V (totalling 0.5V or 10% of the supply !) to the pic at around 3 MHz but slightly non repetitive, the pic was driving a pump with a 50 Hz square wave via a mosfet. OK the things were an absolute disaster but as far as I can tell the actual running of the pic was unaffected, the problems were of a different nature. Of course this is not the sort of thing though you want to find in a product that was "designed" specifically for your needs. It's one thing to let it go in a home one off but in 500 units ?
my own DC to AC driver board had small spikes on the pic power supply even though it has an electrolytic capacitor across the supply and it's only driving a Kohm load of resistors, of course it works fine