Is it neassary to retorque a set of heads after the first warm up.

Today`s head gaskets typically do not require re-torquing. I guess it`s always a good idea though ....

It depends on the gasket being used. Most head gaskets today do not need to be retorqued after a heat cycle, though.
The Flepro Marine's I used said that after the heads are completely torqued down to loosen each bolt one at a time (in the proper sequence) and then retorque each one.

I've never heard of that before. What would be the reasoning for doing that?

I don't know for sure. Probably just to make sure torque on all bolts was the same and that some of the bolts didn't loosen up a little when others were torqued down.

Is a word that means that torquing the bolt properly only occurs while the bolt is turning. So may times you have to put higher force on a bolt to "start" it turning than would be the case if the bolt were already turning in the first place. When the bolt is steadily turning, and you stop turning it when the "torque wrench" clicks, then if you reverse the ratchet on the torque wrench you will not be able to break the bolt loose with exactly the same force you used when you stopped tightening it, you will get the click before it starts to loosen.

Works like this, you car tires are hard to "break loose", but once "broken loose" then it does not require the same amount of throttle to "keep'em spinning" always a just a fraction little less power required.