We used to do a fast realign in the turnarounds as we often only had 30 mins. Dont have IRS on our a/c anymore (different a/c). Im not aware of a requirement to do a full IRS alignment if going into RVSM - I changed whilst RVSM was being introduced. If we had a long turn around 1hr+ we might do a full realign or if there was a noticeable problem with the IRS's e.g. wander. The a/c position will update when you press the TOGA button for Take Off to the Rwy threshold (or displaced threshold if you enter that in the FMC's). They should as far as we are concerned be positionally as accurate as each other (full vs. fast).