Ben, sorry, I don't have an answer. A number of us have looked in vain for a thorough instruction book on how to use a combi, and I am not sure if one exists, but if it did it would answer your question. If you are going to cook with high heat and humidity, I might preheat a few minutes, but doubt it makes much difference. If you are making bread, it might make a difference if you put the dough in a hot humid environment, but other than that, if you are cooking something that is going to take more than 10 or 15 minutes, I don't know if the temp when it hits the oven is all that critical.

If I am cooking with high heat and humidity, I pre-heat. The high heat tends to decrease the steam IMO.

If I am cooking with 100% steam and low temperature..say 220 degrees ie: steamed vegetable, I just put the food in and turn on the oven (I don't pre-heat). The steam fills the oven quickly. Why pre-heat, open the oven and lose the steam? If the oven is already hot, it's harder to get the oven to fill with steam (just my observation).

If I am using zero steam--just convection, I always pre-heat without food in the oven, but of course it doesn't take long.