Coming to a full stop before take off
This question might be somewhat ambigious from a technical point of view - but here goes:
I have been flying a lot lately and have noticed that sometimes the aircraft starts its roll and the engines start to spool up just as it enters the runway from the taxiway, while at other times it comes to a full stop on the runway and then commences take off.
Assuming that the aircraft has not been given a "taxi into position and hold" clearance - is it preferable to come to a full stop before take off or not? Also are there specific airline policies that state either?