I think it depends on what you're currently working on. If it's a new behavior then yes she needs the reward as quickly as possible. As Fodder mentioned, a marker is very helpful for this since you can mark the exact thing you are rewarding (her butt hitting the ground on a sit, for example) and that gives you a bit of delay time to fumble with the treat. Or for a behavior that may not be totally new but is still not fluent. Such as if you've taught her to sit in the kitchen and family room and now you're working on sit outside, use the marker and reward her as quickly as possible for sitting.
If it's a behavior that she is very familiar with and you're working on duration (such as stays or heeling), then gradually increase the amount of time she has to stay or heel before she gets the reward. Variable reward schedules are best for this, so it's not predictable when the reward will come, nor are you always making the delay before the treat longer. So sometimes six second treat, treat, two second stay, treat, 10 second stay, treat, 7 second stay, treat, etc.
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