if you are gonna stereo mic, you can only reap its rewards if you pan....how far you pan will depend on the sound you are after and how it fits with the rest of the tracks and etc etc.......
The "essence" of XY dictates it to be panned hard left and right. Of course, as Gidge mentions, if you want to experiment and adjust, there are no hard rules. But the whole idea of it is to make a natural (and mono compatible) stereo image.
If I'm doing xy on acoustic guitar I ususlly go hard left and right because xy does not produce the widest stereo field anyway, so it can use the help of wide panning.