Precision
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One advantage of the GRIB files is that it saves space over 32-bit binary 
files.  However, the precision of the data must be specified with the usual 
trade-off between precision and space.  The precision can be determined by 
using the "-V" option of wgrib.  The monthly means have one extra digit more 
than the "raw" 6-hourly files (justified by sampling theory). 

The finite precision of the Reanalysis grib files will affect some 
calculations and 

   you must determine whether the GRIB files have sufficient precision
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for your needs.  If you need more more precision, you either have to get the 
data from the spectral sigma files (atmospheric state) or have to arrange for 
a special reanalysis run using higher precision output.

Budget calculations often require a higher precision.  For other purposes, 
the GRIB files may be adequate as the analysis error is probably larger than 
the round-off error introduced by the GRIB format.