OK... the only phone that I had for an extended time that supported fast charging was the Nexus 6... I used it for about 13 months. I subscribe to the 'charge early, charge often' camp. Not because it is technically better for the LiPo cells (it is), but because I never know where life will take me and I don't want to be caught with dead phone.. because I know with absolute certainty that this would also be a situation where I would NEED my phone.
But.... back to the Nexus... After 13 months, with most of the charge cycles being done via the stock QC brick, the battery capacity has only degraded slightly, and certainly not enough to make any kind of appreciable difference to the life of a full charge. All batteries slowly lose their ability to hold a charge over time, it's a matter if you help it along or not. Other than thermally stressing the thing out all the time, like leaving it sitting on your dashboard all summer long, the two quickest ways to speed up that degradation is : a) constant deep cycle recharging - going 100 to when the phone goes dead and back to 100 over and over or b) extended stays on a charger when its fully charged. Both are bad for the cells, the former being worse, but sitting on a charger is still not quite "best practice". It's not "I killed my phone's battery in a week" bad, but if you have a tendency to leave your phone on the charger overnight, it won't last as long as if you took it off once it was done charging.
Besides, if your phone is fully charged, the brick isn't in fast charge 'mode' anyhow... phones usually dial it back around 80 or 90%, and then it dials things back even further the closer to 100% it gets, so whether or not FC is enabled doesn't change anything.