Bob, Yes to all those safety factors.
Relief valves are design to protect components in the hydraulic circuit, by limiting system pressure from exceeding other component limitations. Say you have a pump that can put out 3000 psi, but your cylinders have a rating of 2500 psi. You would then want to limit the pressure to something less than 2500 Psi for longevity. I don't know if you know this or not, but the force required to raise an empty bucket is very low, probably not more than 400 psi. The actual working pressure at around 2750 psi is only achieved when your lifting weight approaches the maximum lift capacity.
Yes, you can cheat the system as designed, but will the steel, joints, bearings, etc tolerate the excess. You might see components with a test pressure on it. That means that product, or one out of a batch of 1000 was tested at a much higher pressure to account for safety reasons. A stamped hydraulic hose at 3000 psi, might stand a test pressure of 4500 psi.
The whole point is that you have to be aware of your system and what you are trying to do with it. You can use it as is, or customize the system for your particular purpose. Most people like to get a little more out of a system if possible. That's my story and I am sticking to it.