Different alloys cast differently. Some older castable aluminum was too porous to be air tight. If it does not say "tubeless" it's foolish to run it as such. They can seal up fine and sit for 3 days and not lose pressure, but go subject it to the heat and stress of operating and it starts to bleed off pressure. If you are not completely intimate with the handling and ride feel of the bike it can end up dangerously low in a short time. If you need to swerve or just roll into a favorite corner with gusto, it'll handle like shit and change your line for you right when you can least cope with it. If it's not tubeless it's likely not to have a safety bead either, and can result in dismounting the tire off the wheel. At that point, you will be in a world of shit that could have been avoided for the price of a tube. Be certain, or risk being road kill.
$0.02