Putting each cylinder at TDC works but it still leaves a little room for the valve to drop a little and makes it a pain in the butt.
As mentioned, start the cylinder up to TDC then feed about 5 feet of 1/4 nylon rope into the cylinder.
Then bring the piston on up until the rope is compressed against the valves and they don't drop at all.
Some use air in the cylinder, but I think that way is more trouble than it's worth unless you have all the stuff to do it already.
The rope is more positive and reliable in my own opinion.