When does my engine need to be set at top dead center?

Ok, so i'm teaching myself to do a head gasket replacement, i have a car manual and i can find a multitude of answers on how to set the pistons on my ford explorer to top dead center however nowhere does it explain WHEN and WHY to put it at top dead center
when replacing the head gasket on my car i know do not have to remove the timing chain/belt. I have already removed the ignition coil pack (instead of distributor) and the intake manifold.
i was wondering whether it is necessary to put the piston #1 at top dead center before i remove the valve covers, rocker arms andamp; pushrods, and cylinder heads, and why it is necessary
what relevance does this procedure have? Is it safe to turn the crankshaft when the ignition coil pack and intake manifold are already taken out?

It only needs to be done if you're replacing the timing chain/belt. That's how you get the crank and cam shaft in synch.

If it's an overhead valve then you're right you don't.

Actually, the manual DOES tell you exactly when you need to set the #1 cylinder to TDC. If the step by step instructions for what you are doing don't include that step, it isn't required.

It's more about reassembly. Before you put the heads back on you set the crankshaft to TDC and if it has over-head cams they must be set so that all the valves are in the correct position relative to the #1 cylinder being at TDC.
If this is a 4.0L pushrod engine it's not a big deal because cam timing is tied to the crank underneath the front cover and you don't touch it when removing the heads. You should however have the heads checked carefully for cracks before putting them back on, the 4.0L engines always cracked and many people do head gaskets thinking this will solve their problem.
Yes, it is safe to turn the engine over by hand, remove the plugs first so that it's easier.

You would only need to be concerned with TDC if you took off the timing chain or had a distributor. The newer cars usually have a crankshaft position sensor. TDC gets the #1 cylinder at the position near when that spark plug fires. In older cars, you would put the engine at TDC and then staticly set the distributor to fire. This would allow you to be close so that the engine would start. Once the engine was running, you would use a timing light to set the timing right where you wanted it… Usually 5 - 10 degrees before TDC. With today's computerized engines, the crankshaft position sensor goes in one place (no adjustment). The spark timing is controlled by the ICM (ignition control module) after receiving inputs from other sensors.