The AIP states at or approaching the holding point. Personally I think that it is unprofessional to call early. I have seen this happen in Melbourne at intersection Juliet when SMC had cleared aircraft X to Juliet who had been on taxiway Alpha and the tower cleared aircraft Y to Juliet who called ready whilst taxiing down Sierra. Aircraft Y had to break suddenly when X taxied in front of it as both thought they had right of way. Why would you call ready early if you still have taxiways to cross? Not a clever thing to do. Rules are there for a purpose - safety. Everybody gets their fair turn.