I add mine to the water I put in, but if you add enough dechlorinator to treat the whole tank after you add water but before you turn the filter on I can't imagine it's going to do anything bad.
I would say, from a noobish stand point this is good.
I add to the buckets + .1ml each ( if a cap treats 50 gal and is 5ml then .1ml per gallon that would be .5ml per 5 gallon bucket, I add .6 to accommodate any error of air in syringe etc)
I perssonaly would never add water straight from the hose for fear of damaging my bacteria or fish but I see the logic, if you do, to add a full tank worth of treatment prior to sticking the hose in... (this creates the additional temperature stress and is another question altogether but just as important)
pre treated water will be ready/safe b4 it ever reaches the tank, post treated carries a risk, however small it may be.