This calculator is handy:
http://www.engbedded.com/fusecalc
It doesn't totally tell you why you might want to set certain bits (it does in summary form).
As a general rule, don't fiddle with bits you don't understand. In particular SPIEN (ISCP programming) should stay enabled.
Also, rather confusingly, a fuse bit is "programmed" (set) if it is zero. This is because the default for flash is for everything to start off as 0xFF (1-bits).
As an example for the Atmega328:
The SPIEN bit is programmed (enabled), as you can see, if it is zero.