The string trick is a nightmare if it goes bad. It's filling the cylinder with string and then turning it to hold the valve up with the string/piston.
To hold a valve closed, you simply have to add air via the right adapter (compression tester) to the cylinder through the plug hole. The top end coming off would not help hold exhaust valves. Once you've removed the rocker, the valve is basically held up and sealing the cylinder. Add air and it holds them closed.
To do rockers, say you don't want to hurt your DD and not do springs only because it's too cold. (wink, wink)... I'll leave it at that. With 1.8's you "should" be ok on stock springs.
