There's not really anything you can lose, just try to keep everything clean. It might me a good idea to take off the loader, gives a lot better access to the valve. If you ever have to replace any spool o rings, definitely remove the loader.
Replacing the 0 rings on the valve is not much different than fixing a leaking kitchen tap (technically also a hydraulic system). Just be sure the system isn't pressurized. Remove the loader, lower the 3 pt attachment or backhoe so they are on the ground and cannot create any system pressure by dropping. Shut the tractor off. Move the joysticks around, depress the nipples on the quick connects on the valve body your working on. Same goes for replacing any leaking hydraulic lines.Your work on the cooling fan/steering washers was a much more involved job.