Peeing in the sample cup, dumping it out, then pour in the sub takes too much time. Certainly more time than it is to pour in an already-heated sub. So if you want to avoid having the nurse banging on the bathroom door, you should get the sub sample up to temperature before entering the clinic.

Its also way too risky to preheat and dump the sample cup with your urine. Pouring in a cold sub in that preheated cup is not always going to get the sample temp within range on time. If you give the nurse a sample thats under-temp, the nurse will then have you submit another sample on the spot. This time under direct observation.