You don't even have to put a ship in it. You can create an empty TF. Problem with leaving it is that every TF takes prep points.
What I do is at the beginning of a turn create a lot of empty TFs at some base. Then I delete them individually when I want to create a new TF somewhere else.
If prep points are short, higher numbered TFs pay the price. So I order it low to high, air combat TFs, surface combat TFs, bombardment TFs, transport TFs, cargo TFs. And leave spaces so on the next turn if I have TFs that can't be disbanded because they're at sea, I can still get the right order on the new ones.
Example, air combat 50 - 54, surface combat 55 - 58 (excluding 56), bombardment 59 - 60, transport 61 - 70, cargo 71+.
So lets say its 1943. Might have something like this.
50 air combat
51 air combat
52 air combat
55 surface combat
61 transport for a landing
62 transport for a landing
63 transport for a landing
71+ various cargo TFs