dhenry:
I have had numerous systems running without decoupling caps. It is one of those things that you don't need it until you need it.
So your systems are as good as your answers here, I did wonder.
The thing is that a seemingly functioning system can be just on the edge of functioning and things like ageing of components can push it over the edge. So to use no decoupling capacitor is just asking for it.
It is rare that there is too much decoupling. The only case I have ever encounters is that some DC to DC converters will not start with too much of a capacitave load, and the odd case of low dropout regulators, most are not that fussy.