I figure that reserve rolls are not optional. It's up to both players to ensure that the game is played according to the rules, and that means that reserves are rolled every turn. If you notice that your opponent has forgotten his reserve rolls, it's your responsibility to say something. If reserves aren't rolled for, it's both players' fault. If it's too late to go back and fix things (ie, the movement phase is already done), then I just pretend like all the reserve rolls that turn failed, and reserves are rolled for normally next turn, but that's suboptimal and outside the rules. If someone is consistently "forgetting" to roll reserves, it's the TO's job to kick him in the *** or kick him out of the tournament.
Locally, we had a similar problem come up with units that were locked in assault over multiple turns. I saw this happen once:
A: "Well that's it for the assault phase. It's your turn."
B: "HAH! You forgot those units locked in assault over there!"
A: "Oh whoops, let's do that assault."
B: "Nope, too late. You said it was my turn."
You don't get to skip the assault phase because your opponent forgot. Similarly, if your opponent is breaking the rules of the game to your advantage, it's your responsibility to inform him. Example: you are playing against a Space Marine player who is rolling his shooting attacks as if his marines were BS3. A space marine squad cannot voluntarily choose to lower their BS. They MUST fire at BS4 or not at all. By shooting at BS3, he is breaking the rules, even if it's to your advantage.
We're playing 40k, not Gotcha! The assault phase isn't optional. Reserve rolls aren't optional. You don't get to ignore the rulebook just because you or your opponent forgot something and it gives you an advantage.