Is not strictly necessary if the kernel or initcpio.conf options where not changed, but it only takes a few seconds for the default target.
The fallback target takes longer because it will include more (or even all - I don’t know) modules.
I do that as soon as I’m noticing that it is active again when a newer kernel has been installed.
Not problematic at all.
It saves a few seconds and the space that the much larger fallback initrd will need in the /boot directory.
I have never had a situation where the fallback image was even needed. 
And: in case it should be necessary, I do know how to chroot and can also do it by means of actually having a bootable USB.
@Teo
it really is enough to remove “fallback” from the PRESETS line