No plan on using attiny45 is there?:
From the manual's errata.
8.2.4 Rev A
• Too high power down power consumption
• DebugWIRE looses communication when single stepping into interrupts
• PLL not locking
• EEPROM read from application code does not work in Lock Bit Mode 3
• EEPROM read may fail at low supply voltage / low clock frequency
Too high power down power consumption Three situations will lead to a too high power down power consumption. These are:
-An external clock is selected by fuses, but the I/O PORT is still enabled as an output.
– The EEPROM is read before entering power down.
– VCC is 4.5 volts or higher.
Problem fix / Workaround ATtiny25/45/85 [DATASHEET] 21 2586QS–AVR–08/201
-- When using external clock, avoid setting the clock pin as Output.
– Do not read the EEPROM if power down power consumption is important.
– Use VCC lower than 4.5 Volts.
I'm not sure what "too high" exactly means or what "power down power consumption" exactly means (I guess power use when off), or if this is a general trend that just happens to be a bit worse in the 45, or maybe only got documented in the 45, or is just a bug in the 45 that doesn't apply at all to others. The warning is removed from errata for later revisions so I guess it was improved or fixed.
I'm leaning toward 3V anyway. It gives a little better control ripple, slightly worse for potential design upgrades but can worry about that if and when. And now there's this warning that makes me a little nervous.