ElCaron:
I am not going to use a ready-made Arduino board, so my question was: Build a DC-Input e.g. 7-12V with a 7805 regulator or similar, or go with a MicroUSB port and some external 5V power source? This thread seems to suggest the latter.
It really comes down to what power you have available, and what other devices you'll be controlling require. For example, if you're controlling 12V devices with your project it's convenient and inexpensive to start with a 12V supply and regulate it down for the ATmega, rather than boosting a 5V source.
If you're rolling your own Arduino you also have the flexibility to run the ATmega328P from 1.8V to 5.5V. You do not need to limit the design of your circuit to 5V.
Cheers ! Geoff