Wikipedia says they're cooked by steaming or boiling. Searching further, I've found plenty of people saying their way is the best way. Assuming they're well-made and won't leak when boiled, why would I pick one method over the other?
I have looked extensively and it seems that whether to boil or steam is determined by region and it is as subjective as the fillings used. See below from this page .
Other variations include using black instead of white glutinous rice, or precooking the rice and filling, or steaming instead of boiling, or even wrapping into a cylindrical shape or a flat paddle shape instead of a pyramidal one.
You may be able to make a decision based on certain factors of your recipe. For example:
- Some recipes call for the rice to be soaked while others call for it to be cooked. If I was using soaked rice I would probably choose boiling as the zongzi may become too dry with steaming. Same idea for other ingredients.
Another consideration would be how well the zongzi are wrapped. If they are wrapped very skillfully, either way could work. However, if not, the filling could come out much easier with boiling.
So, in the end, I would say that there really isn't a really definitive answer. I think I would use the method called for in the recipe as there may be specific reasons the prescribed cooking method was chosen.