Two questions

Jesus!!! All this work.....

Open the engine compartment, finding seacock, screwing many turns, closing, open closet in bath, closing three (or four?) seacocks, closing, open under sink, closing two seacocks, closing. Loosen cockpit tent, open bath step, open door to gas, close the gas, close door, down with step, hook cockpit tent.

Plus all the other packing..... 

Should probably not have said this: I have never closed anything. Actually I am considering mounting through-hull without seacocks. One less thing to break/freeze. The only seacock needed would be cooling water. I have always been thinking of gas valves inside the boat that people are using all day. Would not that be the first place to leak?

But I have only sailed for 30 years 

Yes, I try to check more or less every year. Perhaps discover one I cannot close. Must admit I did not change it.

But one really good idea is to tie a plug to the seacock. One plug for each. If you ever get any problems, it would be that you break one accidentally.

Think! Think one more time. What can actually start to leak. 

I can think of three things: 1) you break it while doing some work. 2) Something breaks it in sea. 3) freezing.

I really think all of these would have benefit without a seacock. 

Have just read that the seacock for Yanmar Saildrive needs 20 turns!!??

But... all this was meant as a question and a provocation.

I happen to think that there is no reason to expect any leakage any place. The only place I am afraid of is the seacock itself, or the throughull. If you remove the seacock, there is better expansion room for the ice to expand.