Voltage regulation for Atmega328p
Did you already calculate the accumulated current the supply has to deliver? If you know that you can calculate your power loss of a linear regulator:

(U_in - 5V) * I = P

The difference between your input voltage and the voltage you want to regulate is the voltage that drops over the linear regulator. If you multiply this by the current you need, you have your power loss.

Now, you can either accept that loss and go with the easy and unproblematic way of the linear regulator or you don't accept that. In that case you would have to find a way to make the output noise of the SMPS small enough for your purpose (which by the way is not as hard as some people think).