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I think this is an interesting one.

So the Chapter Approved is going to introduce a roll off for the first turn. See here:
https://www.warhammer-community.com/2017/08/09/new-matched-play-rules-in-chapter-approved-aug-9gw-homepage-post-1/

The question is: how do rerolls work in this case. If you initially lose the toss, and decide to reroll the die, can your opponent wait to see your new result to see if he, in turn, rerolls his?
But wouldn't this count as "retro-active reroll" which is now forbidden by a FaQ?

Q: Can I use the Command Re-roll Stratagem to retroactively re-roll a dice? For example, if I take a Psychic test, and then my opponent successfully Denies the Witch, could I use the Stratagem to try and increase the result of the original Psychic test?
A: No. You must use the Stratagem as soon as you have rolled your dice, before anything else occurs.

Also, how about rolling for seizing the initiative? Could you reroll to get the first turn and then, assuming you still lost, reroll to seize the initiative?

What are your thoughs on this one guys?

This is why the ITC forbids the use of this stratagem on pre-game rolls.

It's a mess and we simply don't have a concrete answer until GW says otherwise or changes some rules.

This isn't the same as the Deny example. That one, you've passed your reroll opportunity when you accept your psychic test roll and your opponent follows with his Deny roll.

In this case, you're rolling simultaneously. ...Which admittedly isn't covered, though I'm pretty sure most people would be okay with the lower roller declaring whether they want to use a CP first, and then the higher roller getting a chance to also declare a reroll.
Edit: Not that 40K has to/does follow the rules of any other game, but absent explicit instructions, it seems reasonable to use the same logic employed by virtually every other game out there that allows either player to play a card/reaction to the same game event. If both players pass, the event reaction window is closed. If one player reacts, the other player may also declare a reaction in response. In this case, the lower roll player calls a reroll and that reaction allows the higher roller to also call a reroll, if desired.

...OR, you could roll-off to see who rolls first, which of course you might want to CP reroll to go second in, in which case you need a roll-off to see who rolls first for the test of who rolls first, which you might also want to reroll...And an infinite number of roll-offs later the game ties due to time.

 BaconCatBug wrote:
This is why the ITC forbids the use of this stratagem on pre-game rolls.

It's a mess and we simply don't have a concrete answer until GW says otherwise or changes some rules.

To be fair, the only reason we know the rule is because it was previewed in a Warhammer Community article. The full rule will be in Chapter Approved, including the possibility of clarification on re-rolls.

We haven't been allowing re-rolls on it. We've also decided to do away with the Seize roll as well since this system should completely replace it.

At my local spot we don't allow pre-game re-rolls.

We also don't allow re-rolls for random game length.

Basically we only allow a re-roll if the dice being rolled pertains to the action of a player controlled model.