Spot on Denti, why risk the chance of a runway excursion for the sake of trying to "steal" a couple of metres of tarmac.
Even something as stupid as a temperature change will effect your performance by more than 10 metres, I think that for every two degree increase there is a 0.08 difference in EPR roughly on an A320, so say you do an assumed temp T/O with a slightly off ATIS temp or TAT reading then you will increase your T/O length.