Early childhood Chinese language immersion accent exposure
I will go out on a limb and say yes, it is great to get exposure to different accents even if it is not standard so long as the child is told what the standard would be. (Awareness of the differences) The reason being that people you encounter are not always going to speak the standard pu tong hua exactly the same. So when you encounter somebody from the guangdong area for instance, you can adjust your listening to understand when a person says "si" it should be "shi". Oh really chinese has a certain rhythm to it as well. So if somebody asks you a question that is not in a set phrase you know, you can usually take a good guess at what you are being asked. (Not always but sometimes)

A related side note. Not everybody believes in the phonology acquisition cut-off age. 

In Paul Pimsleur's book, How to learn a foreign language *(December 1980)*He defends the idea that Adults are better equip to learn a (second) language then kids are. And that adults can a achieve a flawless native accent. But the thing is you have to consistently work at it. So if you are talking and are misunderstood you have to immediately correct your pronunciation, and the only way to do that is to ask the person you talk to what you said wrong. And correcting the tones. Of course some people have more difficulty than others.