Here is some VBA code that can change the precision of parameters
Dim oPartDoc As PartDocument
Set oPartDoc = ThisApplication.ActiveEditDocument
Dim oParameter As Parameter
For Each oParameter In oPartDoc.ComponentDefinition.Parameters
'Set as exported
oParameter.ExposedAsProperty = True
'Set type
oParameter.CustomPropertyFormat.PropertyType = kTextPropertyType
'Set units
oParameter.CustomPropertyFormat.Units = UnitsTypeEnum.kInchLengthUnits
'Set precision and decimal/fraction
oParameter.CustomPropertyFormat.Precision = kThreeDecimalPlacesPrecision
'Show units string
oParameter.CustomPropertyFormat.ShowUnitsString = True
Next oParameter
Values for precision
kZeroFractionalLengthPrecision
kHalfFractionalLengthPrecision
kQuarterFractionalLengthPrecision
kEighthsFractionalLengthPrecision
kSixteenthsFractionalLengthPrecision
kThirtySecondsFractionalLengthPrecision
kSixtyFourthsFractionalLengthPrecision
kOneTwentyEighthsFractionalLengthPrecision
kZeroDecimalPlacePrecision
kOneDecimalPlacePrecision
kTwoDecimalPlacesPrecision
kThreeDecimalPlacesPrecision
kFourDecimalPlacesPrecision
kFiveDecimalPlacesPrecision
kSixDecimalPlacesPrecision
kSevenDecimalPlacesPrecision
kEightDecimalPlacesPrecision