systematic way to determine best bit mapping policy
For $2^{2n}$-QAM, Gray code labeling often assigns the odd-numbered bits to one axis (I or Q) and the even-numbered bits to the other axis (Q or I) and uses $n$-bit Gray code labeling along each axis. Alternatively, the first $n$ bits could be assigned to I (or Q) and the last $n$ bits to Q (or I). Depending on the structure of the modulator (e.g. 8 bits entering the modulator on a serial line versus 8 bits entering the modulator simultaneously on a 8-bit-wide bus), one method might be preferable over the other. It is important to note that it is not necessary to convert the data into Gray code; just to use the Gray code labeling.  An extensive discussion of this last point can be found here on Google Groups.