Advice / pointers on replacing valve seal

Maybe I'm just lucky, but I have done this many times without dropping a valve. Remove valve covers, rocker shafts and one plug at a time for the air input and leave the air on while removing the retainers, springs and dampers. reinstall as noted above. Now install the plug and go to the next one. This will help to keep the crank from rotating and dropping the valve into the cylinder. Don't like the rope idea, because if the valve should move it can drag some trash onto the seats and cause a pit. As others stated, do them all.