An Import Variable command in a microscript would be executed for each pipetting channel individually. Also each separate Import Variable command generates a copy of the import file into the AuditTrail folder. Hence over time you could possibly accumulate a lot of data in the AuditTrail if each FCA pipetting script command would result with 8x copies of the import file. I could also imagine potential file access issues in case 8 parallel running microscripts would try to access the same file at approximately the same time when all 8 tips pipette in parallel.
A better way to store the z-offset value if we assume the z-offset is specific to a labware definition, is to create a labware attribute for it and write the value into a labware attribute at the script - use SetAttribute() function - and later read that z-offset value from the microscript into the liquid class - use GetAttribute() - function. Please check out the two posts linked below.