Should I update templates I'm not using?

suppose I’m only using Whonix VMs, would I need to update the Fedora template? does it have any disadvantages if I don’t?

Very clear: YES! Or you’re deleting those with:

sudo dnf remove qubes-template-<templatename-version_number> in dom0 terminal

Do you use Whonix for all your service qubes? sys-net, sys-firewall,
What about global services? There are reported issues with using Whonix
as the updateVM.
Also, you may find it useful to have a standard template available, for
use when you don’t want Whonix, and where you need some vanilla
For these reason I would keep at least one other template on hand, and
keep it updated.
Always keep templates updated.

I never presume to speak for the Qubes team.
When I comment in the Forum or in the mailing lists I speak for myself.