It actually depends on the reason for the snapshot. If it's for testing where you plan to revert to the original state, then I it's usually better to do this with the powered off VM. This does not only provide a consistent state, but also saves disk space, because the memory isn't backed up.
If however, the snapshot is created for e.g. backup purposes where you don't plan to revert and also can't afford downtime, then you would certainly snapshot the VM in the powered on state.
André