In line barrel adjuster

[jay clock]

So just to clarify one adjuster either on the gear mech or on the cable is perfectly fine right?

yes, fine.

 

[Slioch]

So your options are, do you fit an adjuster at the point where the cable emerges from the shifter, do you fit one where the cable passes through a cable-stay either on your top tube or down tube (dependent on where your cables run), or do you fit one on an exposed length of outer- cable (which will require splitting the cable).

 

[Ajax Bay]

The cable does not emerge from the shifter - cable is under bar tape.
The option not to fit one also exists. Once properly indexed (ie making sure ferrules are seated properly etc) there really should be no need for tweaks 'on the fly', for the life of the cable. Cables don't stretch.