I guess it is a very smart move, because USB keyboard are VERY problematic in the context of qubes. USB mouse is less problematic.
if you can, reinstall with PS/2 keyboard and mouse is ideal, because, that way you can create sys-USB during install and isolate dom0 from USB devices. You even have the chance to create a sys-net with USB, wich is good to install the USB network dongle.
I have never used this setup, as I dislike “mixing” sys-net and sys-usb, but I think it is reasonably tested and documented by other folks.
As you are on a desktop you can later assign one controller to sys-net and remaining controllers to another sys-usb.