Some engine manufactures specify to set the tappets cold, others say hot. I've not come across an OEM who species an either or. Whilst you have to work more quickly when hot, I've always felt more comfortable with hot setting. My thinking being that, with hot adjustment; when each valve opens into it's combustion chamber, the valve is being set to open nearer to normal engine operating temperatures, when any thermal expansion has taken place in the valve train. Which should give optimum on-road performance, and reduce the risk of a burnt valve head over time. I have a BMW where the manufacturer states to set the valves cold. When the valves on my Beemer are set with the engine totally cold, the engine runs and sounds perfectly. However, I've not been happy with this when having dealer services in the past, and have had a couple of noisy tappets. When I've subsequently checked the clearances (having left the engine overnight with no start up), the measured gaps have varied considerably from specification. I've put this down to the fact that the dealers will drive the car into the shop and will have partially warmed the oil for drain down, before moving on to set the valve clearances. My gut says that if the OEM says adjust cold, then do so with a stone cold engine. If the OEM says hot, I can see the logic. However, I would shy away from doing a cold adjustment on an engine where the OEM specifies only "hot" settings only, as it is difficult to factor in the gap reduction that will take place after the engine comes up to on road temperature.