Are taxiing aircraft to hold short of taxiway hold short markings if not instructed by ATC to do so?
This is one biggest pet-peeves of mine that even old, experienced pilots have missed after years of flying.

Taxi instructions are just that: Instructions, not clearances. Mind you there are some clearances when on the ground aswell, crossing a runway, entering a runway, backtracking, maybe some others.. mostly involving the runway. But a general text like ‘Taxi to holding point Charlie via Golf’ is an instruction.

Ok, so the difference is just pedantic but there is one big caveat: when you are cleared to land, cross, enter on a runway you are guaranteed the runway is, well, Clear and will continue to be so for the ammount of time you need it to land/take-off. 

This is not true for taxi instructions. You are instructed on what path to take on the ground, but the taxiways and route to your runway is most definitelly not cleared of other aircraft (unless we are talking about low visibiliy ops, but just bear with me for now). When taxing you are still supposed to look out for other trafic and try to avoid hitting other airplanes taxing around as much as you can. 

You might be instructed to go via one taxi path and then later told to give way to another aircraft comming from your left. See how the path has not been ‘cleared’? It is not a clearance.

All this talk to sum up: entering/crossing a runway is a clearance. Crossing a (non-mandatory) taxiway holding point is not. Is the answer slowly building up ?

Crossing a runway hold short makrking (two dased lines and two solid lines) you need clearance.

Crossing a taxiway holding marking (or intermediate holding position, or cat 2/3 holding when not in low-vis) does not require one.