MCU fuses used by Pro Mini and ATMega 2560 and power down In context of Pro Mini and ATMega2560 

Fuses, as documented in chip factory doc are complex topics (I have not really used it before), appreciate pointers and advises, to narrow down those that are needed for contexted task and left un-needed in their default modes. 

Which fuses are used within context of the IDE in general, and in specific of software loading via bootloader and ISP? 

Which fuses related to sleep and power down modes, in context that individual chip modules can be set to various power-saving modes via registers. Does fuses play a part in getting the power-saving effect?

Many thanks in advance
The data sheets are the ultimate source. However, that may be a bit overwhelming. I find the below great resources very helpful. 

1) Online Fuse Calculator There are many others.

2) avrdude-GUI
there are many others out there.

3) the .\arduino-1.5.7\hardware\arduino\avr\boards.txt

uno.bootloader.low_fuses=0xFF
uno.bootloader.high_fuses=0xDE
uno.bootloader.extended_fuses=0x05


4) I also read the fuses from any device before I stomp over them. And check into what each mean.

As for narrowing down the features the Online Calculator very helpful to start with and then read into the data sheet for ones of interest.

For the most part there have several questions and answers for low power modes, using interrupts to re-awake. Where there appears to no direct fuses to save power, outside of clock source and speed. Arguably Brown Out detect is a fuse, but that is not directly related to saving power.