For 12/24V (and probably 48V), I would be a true believer of running it ungrounded.
Like in the previous comment, I don't like the idea of having either pole grounded, and thus touching a single wire could raise one's hair.
However, I am also convinced that it is essentially impossible to get a significant shock ad 12/24V (but, one can still make spectacular sparks).
However, a point in the code....
If you run an ungrounded system, then you are supposed to run all switches (and breakers/fuses) as two pole. Thus, in a sense, you have twice as much work getting it all wired up.
In theory, grounding a single pole is supposed to act as a sink against voltage spikes. I'm not convinced this is true, especially when running a small 110/220V system where one can easily overload a single phase. You will find that cheap (auto type) inverters often have problems with a grounded pole.