Hello,
Do you use snapshot-based back-ups for this VM?
If so then leave free space for these - how much depends on how fast data grows on that disk and how long the back-up and snapshot consolidation process takes.
Even if the above is not the case, I would advise always leaving free space so that taking snapshots before making changes to the VM is possible. (You can also re-direct snapshots to another location though:
Creating snapshots in a different location than default virtual machine directory for VMware ESXi an...
Bob
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