Replacing valve stem seals

[KoopaTroopa]

I'm going to attempt to replace my old 200k mile seals with nice new toga ones, I've seen a few writeups on its procedure and feel confident i can do it with leaving the valvehead on.

I dont have compressed air to hold the valves up though, and i'm a little nervous going that route. I've heard you can put rope down in there until its full enough to hold up the valve. Probably a stupid question but why cant you just make sure the piston is at the top for each valve while you work on it?