it's your choice - I stick to the 350* F because I like mine more well done - many swear that the 450 gives them a better "rare" or medium rare
If you are baking a roast and you test the temperature, and it is not there yet. How long do you wait to test it again. Is there a scale to how many minutes to how many degrees go by? I can't bake a good roast yet and each time I make it the temperature goes over. I don't know how soon to check it again.
Probably your best bet is to get one of those electronic thermometers that you can plug into the meat, and set the display outside of the oven. They usually come with a temperature alarm that will go off at the desired temperature. I bought one at the grocery store a couple years ago and works great when your slow cooking meat.