Timing matters are relevant if you want to get the maximum damage out of an Ex Revenge =P And even when right next to the ground, he has to wait for the cooldown of Bardiche to finish and allow himself to drop down. Besides, using an assist to finish an Ex Revenge isn't really necessary - it's more of a waste than anything, since you can already get some hefty damage from the EXR without needing to sacrifice a bar of assist.
I'm not implying that Flare shouldn't be used to finish an EX Revenge (because it should) or that timing is irrelevant inside of EX Rev (I meant once you start the assist combo it does), I'm just pointing out that on the off-chance you decide to use an assist to end it (knowing Flare won't wallrush), you have other choices to end the assist combo to maximize your goals. Much of this rarely happens, but there are times where you just need a little more to end it, and an assist can provide it. Should you realize near the end of the combo that it still wont cut it, you can back up on something to produce some EX instead of Flare (however useful it may be).