It seems that sometimes your experience in some subject is your worst enemy. My last project was based on ARM microcontroller. I started drawing my schematic, and because ARM microcontrollers need +3.3V input voltage I automatically assumed that AVR need it too. So I put inside LM1117 voltage regulator and other unnecessary components. Today my colleague told me that AVR works fine with +5V. I scrapped 50% of the schematic and started again.
Immediately I discovered that +5V circuits are cheaper than +3.3V (for example when you compare MAX232 and MAX3232). Later I found also that AVR may work faster on +5V - 16MHz. On +3.3V it may work with 8MHz only. Not that I need 16MHz anyway for my PRT (Programmable Room Thermostat).