All the time, why? Is that an issue in some places? Most the kids in our school have lunches packed in a combination of stainless and glass to avoid all the plastic softeners that affect early developmental hormonal systems.
I wouldn’t worry about that, it is largely a misunderstanding of the scientific literature. There is a big difference between he optimal ranges, and good healthy growth ranges. The optimal ranges are just for maximum reproduction speed, the reality is these organisms do just fine in much wider ranges.
Almost all bacterial strains used for yogurt making thrive in very similar temperature ranges and are way more tolerant then most people think. For example, people often quote the peak ideal ranges assuming that those are required for rapid growth, they are not. They are just the peak ranges for maximum growth. Streptococcus thermophilus is often listed as 45ºC to
47ºC peak growth range, when really it starts rapidly multiplying at temperatures around 30ºC and dies at around 50ºC. The same for Lactobacillus bulgaricus. Bifidobacterium has an optimal growth range of 37ºC to 41ºC, but does very well from 28ºC and starts to die at 48ºC. Lactobacillus acidophilus has an optimal growth range of 37˚C to 42˚C, but does very well from 30ºC and starts to die at 55ºC.
The post above claims that starting with one temperature encourages one type of growth and then changing the temperature encourages another. This is not correct, any dominate strain will inhibit the growth of other strains unless they have a symbiotic relationship. A staggered start almost never works. This is why fermenting preserves foods, the good bacterial strains prevent new bad strains from growing, they also prevent new good strains from growing. If you want to start a multi-strain culture, you have to pick a range that they all do well at from the very beginning. I often start with a 16 strain starter culture. As you can see there is a large overlap that they all grow just fine in, and that is the sweet spot that most yogurt makers try and hit.
Likely your existing system is just fine for any common probiotic strain you throw at it.
edit: not all strains taste the same, you might not even enjoy the taste of yogurt produced by some of the other cultures. most the cultures we are discussing are thermophilic. there are also mesophilic cultures which product amazing yogurts and grow at room temperature (21°C to 26°C), they don’t need any heat at all. Mesophilic are often thick and cream and sweet, the thermophilic yogurts tend to be a little thinner and have more of a sour sour bite.
Once you go down this rabbit hole expect to be making kefirs kombuchas and more! 
