usually defult 180, 250 setting are volt-ed not really low hence u can push it lower.
so to run more efficiently, clock speed range must be as wide as possible.
180 clock-1500 clock mhz optimal.
-why?
carry x load needs n time.
but if the higher clock reduces n time = power savings.
hence the reason.

now, since u got a wide range, optimally, u set the lowest clock as low as possible.
180mhz-760mv is what i used to do.
why?
since u are doing nothing (idle) the clock will drop to 180mhz (i hope) so it will drain the least amount of battery.
however, if u happen to have 250mhz, you do not want to set the 250's mv too low. because the processor will deem it insufficient and jump to a higher multiple to run, aka 350 or 550mhz.

so what to do?
mhz-mv//////////////usage
180- lowest///////////////////////common
250- not low but sightly higher
350- medium low/////////////////common
600- if available slightly higher
700- low////////////////////////////common
920- medium higher than normal
1200- medium low/////////////////common
1300-medium/////////////////Bursts
1500-low////////////////////////Bursts
@Scavia- u know what... take ur phone and play with it
this is a bit siao...