Via cable or wireless charging once the phone is charged it reverts to trickle charging from what I've read. There are only a few specific reasons you would turn off fast charging:
Wired:
- If the phone gets hot (mine has never gotten hot fast charging, I feel this may also be related to cable quality?). Overheating may degrade battery life.
Wireless :
- If your wireless charger doesn't have a fan, the phone WILL, in my experience, get hot. Overheating may degrade battery life. Get a wireless charger with a fan if possible (one caveat, I haven't seen a wireless car charger with a fan, so if I'm not running the a/c which blows directly on my vent-mounted phone, I usually cable charge).
- If your wireless charger does have a fan, you may not want that fan turning on and off all night while you sleep. This is the only time I turn fast wireless charging off.
I charge overnight, leave the fast setting on for cable and sometines off for wireless (fan noise. Had my Note 8 for 2+ years, fast cable charged overnight every night and heavy usage, battery was still fine when I got the 10+.