Sterile technique is always a relative matter. The precautions required depend on the experimental situation, including the growth media used, the competitive abilities of the experimental organism, the duration of the experiment, and the intended use of the culture. For these experiments the most serious contamination problem is mold. Many common molds grow well on yeast media and compete effectively, over-growing the plates and obscuring those yeast that do manage to grow. Bacteria are less of a problem as most of them do not grow well on these media and, with a little experience, one can easily distinguish them from yeast. Other strains of yeast can be disastrous contaminants if they go undetected. In particular, a red yeast that sometimes shows up can be really confusing, but is distinguished from our red yeast because it doesn't require adenine.
In contrast to short-term experiments, however, sterilizing media must be quite rigorous because storage allows ample time for even slow-growing contaminants to develop. We have concluded from experience that storing media in the cold is actually counter-productive. It does not prevent contamination, but slows its growth. Consequently contamination may go undetected until the cultures are incubated. It is much better to store media at room temperature and detect contamination before the medium is used.