When ever I pull steer lines out I cap and plug them, but I dont think much will come out as there is 2 valves and an orbitrol in between the steer cylinders and the tank. To bleed the front steer system put the truck into LH side mode. Now steer the front wheels all the way out (clockwise all the way IF in LH mode. Will be opposite if you choose RH mode) once all the way out turn the wheel one more full turn against the pressure relief or "lock". Then steer the wheels all the way in to the 0 degree or home position (Counter Clockwise all the way) and steer one full turn against the lock. Now cycle the mode change from side mode to forward reverse mode and back again. Check front wheels both stop at 90 degrees to the truck. If wheels are not around the same angle this procedure may need to be carried out a couple times. If your still have trouble with front wheel alignment the sychronized oil between the front two steer cylinders will have to be bleed manually OR you have a bypassing steer cylinder. (The Front line on the RH steer cylinder is plummed to the back line of the LH steer cylinder. Essentially when the RH steer cylinder goes out it pushes the fluid from the front of the piston to the back of the LH cylinder so that both steer cylinders move together and are in "Sync" with each other. If the amount of fluid between these two cylinders change you will get a front wheel alignment problem.)
The only lines you have to be carful about leaving open are the return lines for the valves and motors. most of the time they run to the bottom of the hydraulic tank with no check valves. These lines if left uncapped WILL drain the entire hydraulic tank onto the ground. Just get some JIC caps and plugs and youll be fine.
How about when you changed any steer lines better keep to keep it up the mast, and after bleed the lines at the tank
The point to raising the mast is to lower the level of hydraulic fluid in the tank. This is to prevent the tank from siphoning when the suction filter is removed. (Big yellow spin on). This is not necessary when changing the return filter.
So really do you HAVE to raise the mast all the way? No, but it will probably make a bigger mess. (But good luck not making a mess when changing it anyways because of where its located in the truck.)