A family member took her compact SUV for its 100,000 mile service recently, and the service manager attacked her with the dreaded "fluid flush gauntlet". He showed her samples of her transmission oil, power steering oil, brake fluid, and coolant, and tried to sell her about $500 worth of fluid flushes. He also tried to convince her to change out her rear differential fluid, which is rather expensive, because it comes from nearly extinct whales.
This is a scam. It is pure profit for the shop.
You should not get any fluid changed on your car that is not recommended by the owner's manual, unless you have special circumstances. More on those below.
She checked her owner's manual, and while the transmission fluid and coolant were recommended, there was no mention of power steering fluid, rear differential, or brake fluid.
And here's why: all three of these systems are sealed. In the power steering and brake fluid systems, the fluids are primarily hydraulic working fluids, although they do have some lubrication functions. If the systems are still well sealed, and are working properly, you may not need to touch them for more than 150,000 miles.
Part of the trick is to show you that the fluids are "dirty". This is scary, but not necessarily meaningful. I asked a guy I know, who is involved in the design of power steering systems. He told me that power steering fluid picks up carbon black from the hoses, and that you can't really tell a 5,000 mile fluid from a 50,000 mile fluid based on color. His professional advice was, "don't touch it unless you are getting noise from the system or hard steering". And if you don't want to change it, he recommended using a turkey baster to remove as much from the reservoir as you can, replacing the rest with clean fluid. "The flushes the service places sell are junk!" he said,"A waste of money."
So when should you change your fluids?
If the owner's manual says it is time to do it, or if something is wrong, or clearly contaminated. If large particles are obvious in your fluid reservoir, or if there is water in the oil, or if the system has failed in some way, or is making strange noises.