[Nux]

All my VMs have a separate /boot and ext4 as / (xfs would work, too).

With this setup - using KVM, mind you - I can change the disk size of the VM and resize the / filesystem inside by just using resize2fs, no need to reboot, umount or anything else.

[brazzledazzle]

The impression I got a long time ago was that it's potentially unsafe. Is that old information?

[Nux]

I have not read that anywhere, I did it many times without data loss or other issues. You just need to use a relatively recent distro (Centos 6 +).

If you use xfs, watch out, you can grow a fs, but not shrink it - which is why I use ext4 when I want more flexibility.