beaslbob - you don't do water changes, so your comments are out of context.
Even though you only top off you could be at risk if you're on a municipal water supply that may switch to chloramine (which does not dissipate like chlorine).
Byron - I was not baiting you, merely communicating a Seachem Tech Support recommendation as I was reviewing their forum. I appreciate that you only use conditioner for the amount of new water added and not the entire volume. But how many times have I seen you write 'simply because you get away with something doesn't mean it's best for the fish'. Besides, you have heavily planted tanks - what about tanks that aren't planted? What about newer tanks not yet cycled or tanks that may be cycled but not yet established? ...
I don't think the recommendation is really an attempt to push more product, but more likely simply being a conservative approach to better ensure the success of a novice fish keeper.
Since they have tested the product at 5 times the normal dosage and found it to be safe with no ill effects why not recommend dosing for the entire volume for a typical water change, better ensuring that chlorine/chloramine converted to ammonia is bound and detoxified until plants and/or bacteria can process (oxidize). As to the detoxification of nitrites and nitrates, Seachem freely admits that this was not intended or designed, but later discovered. Surely that merely enhances rather than diminishes the usefulness of the product.
As to Seachem Flourish, my label says 'Refrigeration after opening is recommended but not required.' Also, in a separate thread on their support forum, Flourish was said to have an indefinite shelf life.
In closing, I was not looking to start anything, merely pointing out a manufacturers recommendation - or at least one of their Tech Support personnel's recommendation. Clearly dosing for the entire tank volume for the typical water change may not be absolutely required. However it may be the most conservative and safest approach when using a drain/refill device.
Footnote: I have no affiliation with Seachem and have a non-chlorinated well system (not a municipal water supply). I am not paid to make comments or start trouble.