The MAC data from either trimsheet or computer calculation is only as good as the data going in and the design of the system doing the calculating.
The "accuracy" of the IU and/or MAC, likewise, is highly dependent on a variety of matters.
In general, if the nominated CG is within 5-10 mm for empty (depending on aircraft size, geometry, and scales being used) you are doing pretty well, considering all the avenues for error in establishing the empty weight data. For instance, I can recall investigating one Type's dreadful history for an operator many years ago .. turned out that inappropriate jacking procedures were causing significant unexpected scale indication errors.
As the load goes on, the error potential expands and one ought not to be expecting too great a loaded CG accuracy.
I see people, from time to time, plying CG numbers which are away with the fairies in that the implied accuracy is totally unsustainable.
In respect of the present question, and acknowledging that I don't have the documents etc to hand, I would suggest that it doesn't matter whether you round up or down for the FMS.
So far as stab trim is concerned, there is more than enough fat in the certification FT arena to ensure that one need not worry too much about round off effects ....