the theory behind dechlor is that the chlorine and chloramines in your tap water kill the bacteria that you are trying to grow, now at the moment you don't have a lot of bacteria but you will have very minute numbers of them, there is a chance that using water that had not been dechlorinated just killed off the few bacteria that you had which would mean that the cycle wouldn't start.
now i actually think this is fairly unlikely, without getting into a whole debate about the validity of dechlor, there is certainly some debate over weather it's actually needed or not the main thinking being if there are some bacteria in the tap water in the first place then they must have developed immunity to chlorine/chloramine or they would have been killed off and never make it into your tank in the first place. however because dechlor is cheap, easy to use and does some other things like break down heavy metals most of us decide to use it anyway as a bit of a safe guard.
We fairly regularly get people cropping up on the forum saying they just did a water change without dechlor and panicing about what will happen, I can't think of a single case where there has been a mini-cycle or a full cycle attributed to this happening.
So you have a choice, you can leave things as they are and it will probably be OK but there is a theoretical risk that you've crippled the cycle before you even started. Or you can go for the safe side, drain the tank and do a 100% (or as close as you can get) water change using dechlorinated water and then top the ammonia back up to 5ppm.
It's up to you really, while I don't think there's any real risk I'd probably do the change to be on the safe side anyway.
