Deformation quantization generally produces families of `cohomologically equivalent' quantizations of a single physical system. We conjecture tha tphysically meaningful ones i) allow enough `observable' energy distributions, i.e. ones for which not pure quantum state has negative proability and ii) reduce the uncertainty in the proability distribution of the resulting quantum states. For the simple harmonic oscillator, this principle selects the classic Groenewold - Moyal (Skew-symmetric) quantization of phase space while for the free particle, for which there is no real quantization, all cohomologically equivalent quantizations are equally good.