cot caught merger!! which to follow between talk[tɔ:k] and talk[ta:k]?

I guess this question could be answered by North Americans.
As you have experienced, in the mid-western part of America, they pronounce the volwel [ɔ]in talk, thought, caught same as [a] in cot or doll, but in eastern part of American, they distinguish the two.

I even saw two Canadian sisters whose elder pronounces talk[ta:k]. and the younger does talk[tɔ:k].
According to a statistics, the ratio between cot-caught merged people and non cot-caught merged people in the states is 43:57 which shows a slight more preference of the distinguished trend.

Does American English have any standard form? They don't seem to have any formal language and make the difference with many dialects. When foreigners learn English, which state's language is appointed or how do they appoint the pronunciation standard to teach foreigners? We normally learn talk[tɔ:k], but when we meet many Americans, we can hear talk[ta:k] a lot as well.

As foreigners, which do we have to follow when we learn English? A good opinion of any North American would be highly appreciated.

Most Americans who are "national" (like people who read news on TV) speak with a basic generic midwesten accent. But there is no "official" accent in America.

If I was to advise someone how to pronounce English, I would advise them to emulate the people they are trying to communicate with. If you are in New England, try to sound like them. In Texas, talk like they do. The point is to communicate and I can't see the point in trying to adopt an accent that is deliberately "foreign" to those you are speaking to.