I have a 256-byte file (0255.bin) containing the bytes 0x0 through 0xFF. I wrote a test app to read that file into a buffer, print the decimal values of the bytes in the buffer, use MultiByteToWideChar followed by WideCharToMultiByte (with lpDefaultChar equal to NULL) on the buffer, then print the decimal values again.
I did that for the ANSI, OEM, and THREAD code pages.
In all three cases, the before/after decimal values were identical; i.e., the decimals 0 through 255.