Pronunciation question, please help. thanks?

Well it depends on the pronunciation and generally the accent of the speaker. Some say CH and some say T. There is no right or wrong but there is the notion of appropriateness in this. For example, DONCHU is a bit colloquial and it is used in informal conversations. Because it is easier to say DONCHU than DONTYOU (you have to make a small pause for pronouncing /y/ after /t/ so it makes it a bit hard) Another example is LAST YEAR. some say LAST YEAR and some say LASHT YEAR (usually British people say the latter). When I personally talk to my teacher I say DONT YOU and I'M GOING TO, not DONCHU or IMGONNA!

I don't think that the way you're spelling it into your mind is visible to your interlocutors.

If you are answering formally, you should pronounce it don't You, however over the years pronuncuation has slipped, and we all talk a bit slangy now. I say dont chu but if i was talking to an adult or being polite i would say dont you! Hope I helped xx