TeslaIaint:
According to the data sheet for the following switching regulator, no additional external components like decoupling capacitors are needed. Can anybody confirm or refute this? OKI-78SR-5/1.5-W36-C Murata Power Solutions Inc. | Power Supplies - Board Mount | DigiKey
If you look at the datasheet there can be some ripple or noise, it's the R/N(mVp-p) column on the FUNCTIONAL SPECIFICATIONS SUMMARY AND ORDERING GUIDE table of the second page. The 5 VDC version has a maximum of upto 75 mV peak-to-peak, or about ±37.5 mV. Still it's a small swing, only 1.5% of the nominal voltage and the ATmega ICs used in most Arduinos can tolerate upto 6 VDC on their VCC.
However, I have seen some self-contained switching units that include enternal filters that have less ripple. Here's an example. According to the that module's datasheet the maximum ripple is 50 mV peak-to-peak.