If booted to linux and then rebooted, fastboot will not affect it. That's why you will not have a problem when reboot from linux. If on windows, you reboot, fastboot will by its function like hibernation, will store some data at shutdown; and that will affect linux startup from a windows shutdown.
Nothing changed. And nothing to do with grub.
You will also need to watch out for hiberfile in windows, not just fastboot.
Disabling hiberfile will also disable fastboot.