Well, you would not have any real need to test your tap water after you added the dechlor. Adding dechlor to your tap water before you test it will not provide you with any helpful results. However, you must add dechlor before putting chloramine tap water in your tank, since chloramine is very toxic to fish.
But what of the tank? The manufacturers of the dechlorinating agents make claims about how their product would or would not effect various tests. On the link above, I tried to include those claims. It is an interesting point - if there is a pure dechlorinator such as thiosulfate(ie: not an ammonia binder like amquel or ammolock,) in the tank water, will it prevent the ammonia test from generating monochloramine which will turn the reagent color? I don't think it will prevent the test from changing color, since I assume the test kit reagents are present in a relatively overwhelming amount in that test tube, to ensure that all the possible tested for compound reacts.
As for the water treatments that also claim to detoxify ammonia (the ammonia bindeers like amquel and ammolock), one manufacturer claims that its product does not interfere with the salicylate test (ie: ammonia still shows up) but that its competetors product does. I have not confirmed these claims with independent tests.
