The FSB strap gets auto adjusted when the user selects a specific memory multiplier i.e when the [6:5] memory multi is selected the strap gets set to 333-FSB etc however there are a few memory multipliers that operate on more than one strap so having the option to manually select the strap sometimes comes in handy if the user is having stability issues . . .
vNB (northbridge voltages) do not normally need to be adjusted from [Auto] on a modern LGA775 mobo as long as one doesn't exceed 400MHz-FSB with the processor running using its native multiplier, tRD (aka Performance Level) left on [Auto] and the memory running roughly within spec (i.e under DDR2-1066). . . once you exceed 400MHz-FSB or manually adjust tRD to a lower value or manually set the processors multiplier to lower than stock (forcing up the NBcc MHz) or run the memory at superfast frequencies (or a combination of all the above) then you need to manually adjust the vNB . . . the vTT setting is "linked" with vNB so once that comes off [Auto] I found you also need to start farting around with vTT . . .
All the settings in the BIOS are "fine tuned" by a professional BIOS engineer, and they do a pretty good job . . . there is no problem manually adjusting each and every setting yourself but that normally comes at a much later stage in the overclocking process . . . leaving things like vCore on [Auto] is fine for a basic overclock, in some cases the BIOS will "Over-Volt" the vCore so by manually fine tuning it yourself you can shave a little volts & heat off the final clock . . . on some BIOS revisions the board will cap the vCore limit so you have no choice but to switch to manual over-ride to get the processor stable but this scenario is more rare these days!
Messing with PLL, vNB, vTT, GTL Refs is not a simple business and really is better left until nearer the end of the overclocking process . . . in the case of someone gunning for 500MHz-FSB, DDR2-1200+, 4.0GHz processor +, 700MHz NBcc+, low tRD and other uBer high OverClocking parameters then you will almost certainly have to step in and switch to manual override as the BIOS will almost certainly not be able to deal with a plethora of non stock settings!
There are two types of overclockers . . . one who wishes to get a good speed boost from their system with minimal fuss and without weeks or stability testing and the other is more of a die-hard geek who wishes to hand adjust every single BIOS option they have access too . . . personally I found the best rule of thumb is "K.I.S.S" (Keep It Simple Stupid) and "Less Is More"
. I always thought that as you increased the FSB to a certain value you also needed to choose the right strap to loosen the internal latencies/increase clock speed of north bridge so as to make the system more stable and thereby upping the north bridge voltage aswell in the process. 
.
