Whenever I use the oven to prepare frozen French fries, the instructions tell me to pre-heat the oven to X degrees, and then put the fully frozen French fries inside when it's ready. But it's often a waste of time to stand around and wait for it to slowly get ready, so I tend to put them in before it's shown with the light indicator that it's at the right temperature. Also, I take out the French fries from the freezer at the same time as I start the oven, and spread them out on a thing, so they are ready to go into the oven. Maybe this make them thaw a little bit. Since the times specified on the package never seem to be accurate at all, it doesn't seem like it matters. But maybe it does? Why should I wait until the very last minute to take out the frozen French fries from the freezer, and put them into the oven only when it's fully reached the temperature specified on the package? Is there something about doing it in my "impatient" way which fundamentally ruins the food? Or have they simply timed it "directly out of the freezer, immediately at this degree" at the French fries research lab? 
For fairly forgiving things like french fries, I've never found it makes a difference.  Typically I turn the oven on, get the chips (which is what we call them here) out of the freezer, and the baking sheet out of the cupboard, put the chips on the sheet, put the rest away, then put them in the oven. Maybe 2 minutes of preheating.  They take maybe 2 minutes longer.  This is with a modern electric oven, that reaches the set temperature in about 10 minutes. It may use the grill element to start preheating, but it's not a fan oven. It saves a little energy, but not much, as for short cooking the majority of the energy is in preheating. Most cook-from-frozen things are probably OK this way, as they have to defrost and cook, and defrosting will start nicely at lower temperatures. I wouldn't do this for baking, or for anything thick, so not pizza (even pre-made frozen), and even chunky potato wedges are marginal (I'd be cooking those from raw room temperature potatoes). 