I usually power my AVR breadboard prototypes from a computer USB port. I know the PC power is quite clean, so I typically don't bother with decoupling capacitors. When testing picoboot at 3.3v, I encountered problems (occasional out instructions that didn't change pin state). Swapping the AVR didn't fix it, so then I tried adding a .1uf cap, and the problem went away. I was getting the 3.3v from a USBASP, which uses an AMS1117 regulator, but surprisingly doesn't have any cap on the regulated side.
So if your power source has caps on its output, you should not need any decouping caps for your AVR circuit. If you have them, it's probably a good habit to get into using at least one anyway.