It depends on what causes it to curdle, I'd think.
Curdling occurs when the proteins in the milk contract in respond to acid or heat. This is no different from watching the white of an egg turn hard and opaque when it fries. (Enzymes can also cause curdling...rennin is a good example, used to make cheeses with little/no sour taste)
I would suppose that the more acidic the yogurt, the more likely it is to curdle. Home made yogurts become more acidic the longer they are processed at that 100F to 120F temp. If making home made, heating the milk too high will also cause curdling.
The "sour" of yogurt is lactic acid formed by the bacteria. The acidic environment prevents allot of the "bad" bacteria and molds from growing. Yogurt has a characteristic "sour" odor (open a pint of plain yogurt and take a whiff).
From my very limited experience with yogurt and beer making the rule of thumb I've heard is; if it smells horrible, it probably is horrible: it's bad. "Sulfury", "rotten egg" or "roadkill in the summer" horrible...not just like sour milk (because yogurt is soured milk)
Homemade may be a little more sour smelling, but if the odor coming off it can curl your toes, and/or if there is evidence of gas bubbles and/or colorful or black molds...dump it. (Yogurt can get a white mold on top...this is OK, I've been told. Just scrape it off.)
The rule that tops them all, with regard to food spoilage is :"when in doubt, throw it out."