Yes you will need to have the engine at TDC for the cylinder you are going to replace the valve stem seals on. The reason for this is if something should happen with the pistion down the hole you could drop a valve at that point you would be forced to pull the head off.
You can pull all the spark plugs out after you have removed the rocker shaft assembly to make it easier to turn the engine over. On removing the rocker shaft assembly start by loosening the two bolts in the center first and then working outward from there loosening the bolts a little at a time like one to two turns then go back to center and repeate again. I keep all the push rods in order so they go back in the same lifter and on the same rocker arm too. Good luck