My thanks to all, your answers most helpful and interesting. Also, following pigboat's story, I shall decline all offers of pistons with low temperatures! I have read those stories of draining the oil when it was hot and then thrawing it out on the stove the next morning, I didn't realise how true they were!
To sum up: Given the time that it takes from Start to turning onto the departure runway, you always have enough time to warm the engines through. The last part of the question is - following Start, at what stage are you comfortable that the engine/s are stable and you don't mind asking them to get you off the ground? OR must you presume that an engine can fail at any time for any reason and NEVER take it for granted? Which, I presume, is the correct view!