Where tracks have them (Premership and Glasgow that I know of) they are supposed to be used according to the rule book. An instance where a referee suits himself was at Wolverhampton last Monday - Nicholls still gardening off his machine when the two minutes expired and allowed to continue, Bathelor off his machine two minutes expired disqualified straight away. Referee was Wayne Jarvis who for some reason has been allocated the British Final whilst still technically training, i.e. First year running solo.