So it depends. For example some legacy apple stuff had a bad DHCP implementation where it would try to hold onto an IP address it had before.

When there’s one DHCP server with a reserved ip it won’t assign that ip to the wrong device. (Unless you’re running some buggy software that takes your configuration as suggestions)

Where the advice to set it anyways comes from scenarios where that DHCP server goes down for long enough that everyone starts self assigning addresses. It’s a real hassle to find the correct system when that happens.