Is there a reason to be makpid to pronounce a cholam in the proper way, “Oh”, and not the yeshivish way, “Oy”?
ALso, in general, is it important to be very careful with grammar in davening? (e.g. milera or mileail, sheva na sheva nach..) It seems that most people (Americans at least) are not.
Answer:
Because there is a variety of pronunciations for various vowels (and consonants), and because we cannot know which is the right way, each person should pronounce the words according to his custom and the custom of his fathers. Concerning being particular with grammar, this is certainly an obligation, which applies not only to Krias Shema but even to other parts of the prayer service.
Sources: Shulchan Aruch and Rema, Orach Chaim 61:21-23.
