Boiling leaves and then adding the tinted water will obviously tint the aquarium water quicker than waiting for the leaves in the tank to do this, but I personally would not do this for a couple of reasons. Primarily, leaves not only release tannins, they have incredible bacterial benefits. I have been told that boiling them can remove this benefit, whereas placing the dried leaf in the tank and letting it slowly decompose adds the bacterial benefit. If you add the tinted water, you will need to do this at every water change. And, the more leaves you have lying on the substrate, the more natural the aquascape will appear to you and the fish. Many creeks and streams in the tropical areas where our fish come from have a thick litter of dead leaves covering the substrate. This darkens the substrate which fish "expect," as well as providing a natural substrate. And the tannin and bacterial benefits.
EDIT. FallenPepper's subsequent post reminded me of another reason not touse the boiled leaves water...too rapid a pH change is more likiely. With leaves in the tank it is a gradual process, somewhat held in check by GH/KH if these are significant. But even without, the pH will be a more natural shift.
You do need a lot of leaves. Depending upon how quickly they decompose, you will be adding more periodically. You want to be doing weekly partial water changes of 50-70% of the tank volume, which will obviously remove much of the tint. A couple of leaves boiled to keep the tint much the same at water changes might be worth it, but I would still have a good layer of leaves and maintain it. The rate of decomposition varies, and you can leave the leaves and let them completely decompose (adding more periodically of course) or remove them when they become sort of skeletal, that's up to you. I tend to leave them.
