One needs to keep in mind that the datum position is entirely arbitrary - no more significance than a convenient place to hang the end of the tape measure on ie your datum preference is just as good as Fred's. Hence, any FS reference must always include the datum used otherwise it is a tad useless. It follows that, once chosen, the datum needs to be applied consistently.
A common datum trap is with trimsheet loading systems. Most of us define a trim datum somewhere inside the envelope, regardless of what the OEM might have defined. Generally, for larger aircraft, the OEM defines two datum positions - one for FS reference (which will be wherever that OEM fancies) and the other (often referred to as the trim datum) for loading calculations. Even then, a non-OEM trimsheet designer may well choose to use yet another datum for his/her own idiosyncratic reasons.
The best options for on aircraft datum positions are those which use an easily defined reference .. which is most aircraft. The DC9 type approach is another good fix where a plumb line sits over a grid to define long/lat level.