Earlier quoted context omitted.
I was told to spit out the first few sips.
Traditionally it's the job of the one preparing/serving the mate (the cebador) to take the first sips until it tastes right. So it's entirely up to you if you want to spit it out. Just make sure that by the time you offer it to someone else, that it's tasting good.
Have there been any studies on disease transmission in cultures that partake of drinks in this communal manner?