CrossRoads:
LiPo are more like 4.2V when fully charged and 3.7V when discharged.
If you running at 16MHz, then full performance is guaranteed down to ~3.8V.
If you run at 8 MHz, it's less. So you coul run from a LiPo directly and not use a regulator at all.
I have an RF remote control that does just that - 8 MHz, 1000mAH LiPo, MAX1811 to control recharging the battery from a 5V source.I am testing a '2560 design right now using 3 AA batteries. I have a 24V to 5V DC/DC module on the board, but don't have the right female cables yet to power it up.
Running from battery with no regulator is fine as long as the battery voltage is in the correct range.
Setting fuses is a really hard ask for new Arduino-compatible experimenters in a workshop. Do you know, is attaching an 8MHz crystal like 8.000 MHz 8 MHz Crystal HC-49S Low Profile in the place of the 16MHz crystal more-or-less equivalent to asking it to use it's internal clock, or is the change of fuses also needed to get other aspects of 328 behaviour to line up correctly with the 8MHz clock speed?
It would be great if I could just plug in an 8MHz crystal to the circuit, then use a 'board' setting in the Arduino IDE which would assume an 8MHz clock in its defines for Serial interrupts etc. Then I could use that setting when programming a simple 8MHz ATMEGA328 circuit which could be programmed directly off 5V but run down to a <3.8V supply on a battery without changing anything (assuming the project doesn't mind timings all going astray as per PaulS' comments).