Some comments here to add to context, although this isn’t in any particular order…
Older releases came with the default accounts/passwords “ubuntu/ubuntu”, and later also “nvidia/nvidia”. Later on units had to start shipping without default name and password (part of which is due to new laws in California, but so many people get hacked when they leave default name/pass that this was just a good idea). The “first boot” code is designed to ask for new name/pass info, but there have been several circumstances where it didn’t work as expected for one reason or another.
Development kits do arrive with installed software, but depending on how long the unit has been in storage you may not know which of the releases you received, and it is still a very good idea to start with a fresh flash. Even if everything had worked out there would still be a recommendation to perform a full flash.
Regarding earlier releases, these did not use the Ubuntu package mechanism for the NVIDIA-specific hardware drivers (and these drivers are what turn a purely Ubuntu distribution into what is referred to as Linux for Tegra…“L4T”). If you have a more recent release, then you will find you can do some updates from within the Jetson which older Jetsons would require a full flash to complete (well, not older Jetsons, but really Jetsons with older software releases).
Serial console is something you will definitely want around if you get around some of the issues of video configuration or other bugs. For example, monitors and networking can fail, and since serial console has almost no dependencies, then the system will still work through this (but if there is no login account, perhaps not…although first boot configuration can be completed through serial console).
Do note that if you choose to flash that VMs usually fail due to not holding the USB as the Jetson will repeatedly disconnect and reconnect to USB during the flash. VMs can be made to work if they are configured to always get the Jetson’s USB during flash. If you have a native Ubuntu 18.04 host PC, then this is the least frustrating.