Assuming your location is Europe and/or you are planning to operate in European RVSM airspace, you will need to get RVSM approval from you regulator.
More information here:
European RVSM Approvals | EUROCONTROL And here:
http://www.icao.int/ESAF/Documents/RVSM/jaa_tgl6.pdf
1000ft is not much pressure difference at high altitudes so a well functioning altimetry system is critical. For peace of mind, fly over one of the European Height Monitoring Units every now and then so that if the system gets out of calibration it will be noted by Eurocontrol. They will notify you if your altimetry system is drifting too far off.