Most apps under raspbian do not use gpu memory - only apps that support gpu acceleration.
Video decode in chromium benefits from having 80M.
Higher will make no difference (and if you go too high the arm may suffer when running multiple applications, or with many tabs).
omxplayer can need more than 64M with 1080p streams with many reference frames. 80M is probably fine for that.
Kodi is the other major use case that requires more gpu memory. 128M is considered a minimum but 256M is recommended on a Pi2/Pi3.
If you are not using Kodi it is unlikely you'll need more than 80M.