Adding a small amount of yeast, such as 1/8 tsp, to your bread dough along with your sourdough starter can be a good idea if you want to ensure a reliable rise in your bread. Sourdough starter primarily relies on wild yeast and lactobacilli for fermentation and rising. However, the activity of wild yeast can sometimes vary, leading to inconsistent results. By adding a small amount of commercial yeast, you can provide an extra boost and increase the chances of a successful rise.
Keep in mind that using commercial yeast in combination with a sourdough starter may affect the flavor profile of your bread. Sourdough breads are known for their distinct tangy taste, which comes from the fermentation of lactic acid by the lactobacilli in the starter. When you introduce commercial yeast, it can affect the balance of flavors to some extent.
If you prefer a stronger sourdough flavor, you can rely solely on your sourdough starter and allow it more time to ferment the dough. However, if you want to ensure a more consistent rise, adding a small amount of yeast can be a helpful strategy.
Experimentation is key when it comes to baking, so feel free to adjust the yeast quantity based on your preferences and desired outcomes.