Powering the Uno in a non-standard way

I know the Uno can be powered one of three ways (barrel connector, usb or Vin). I'm working on a project where there is a need for an external 5v switching voltage regulator. Can I power the Uno by providing +5v external power to the 5v pin on the power header? I know this is suppose to be a 5v output, but I also have seen some other Arduino form-factor compatible boards which allow this connection. If this is possible, it would provide the most efficient use of my battery power as the external switching regulator is considerably more efficient than the built-in linear regulator on the Uno.

It is possible but there is a little difference as the power does not go through the regulator - so spikes can do serious harm.

Thank you. Seeing the schematic helps. The switching regulator feeding the project is the 5V DE-SW050 module from Dimension Engineering (http://www.dimensionengineering.com/DE-SW050.htm). The battery is a small 2 cell Lipo pack. The battery/regulator setup will also power a bunch of sensors, but no heavy load things like motors. Given this setup, do you think some little capacitor in the circuit might be prudent before feeding the Uno?

If using a switching regulator you want one with good performance (output noise low, never spikes above 5V). There are many DC-DC switching regulators these days with adequate specs. Remember when you're on USB you are ultimately getting the 5V from a computer's switch-mode power supply.