Body inflation is already somewhat dangerous, and using helium only ensures that a dangerous fetish is more expensive. If you must, be extremely careful to not use helium (or air, for that matter) that is under pressure, from a tank. You risk internal organ damage, intestinal perforation or air embolism, which can lead to death.
I think the only thing that you must be aware of is to know the limits of your belly, what I mean is that if in the moment you’re inflating yourself and feel like you can’t keep doing it, just stop. Why? Because that can make your colon literally explode and may result in your death, or in a serius body injury.