If you want them to stay on target, you have two general options.

My IWT, is not a simple keybind to all toons.
It is a mapped key that sends IWT to all members of my melee group.
And then I have another mapped key with two steps to enable/disable this key.
So I can toggle my "spam" IWT on and off.

I also have IWT for all toons, on its own key.

If my spam is disabled, I can push the single IWT to everyone...
And break the movement immediately on the healer/active toon.
That will move my DK's (any toon whose movement does not break the IWT movement) towards where the target was when I pushed the key.
They won't over run the target, nor adjust to a new position if it moves.

Similarly, without a toggle, but with a "spam" set up...
You could push the key once, and wait.
Once they're in melee range, and have stopped moving.
Then you spam the shit out of the key.
And since they can hit the target where they're standing... they won't need to move towards it.
So there will be no movement, until the target moves.

For PvE, I use the second method much more so than my toggle.
For PvP, I just spam like crazy... probably spamming 2.5 presses per second on average.
I also has IWT on press only, not on release.

With PvP and the toons circling the target like crazy.
I haven't found an appreciable difference on a target dummy, when comparing DPS stationary vs DPS circling like crazy.
Either is close enough to the other.
Plus in PvP, the mob/player will likely move, and if you're not spamming you won't stay with it/him/her.