The fluid that NEEDS to be changed is the fluid in the lines and the fluid in the caliper. That's where you are prone to see degradation / air bubbles. So, while the reservoir may look nicer, you can still have really crappy brake fluid where you NEED it. You can empty the reservoir and refill before you bleed to speed the process, but just emptying and refilling is a waste of brake fluid.
Clutch - same thing.
Power steering - safe easy idea. Emptying the reservoir gets about 75% - 80% of the system volume and it's pretty non-stressed fluid. We do the same after every track event.
Why the difference in these three? The power steering system is the only one with a pump that CIRCULATES the fluid throughout the system, so it's the only one that will actually evenly distribute your nice new fluid.