I’m not an expert, but here’s what I know… you DO need to have the sauce and jars hot when putting them into the boiling water – otherwise they can crack. I’m not sure if you would over process them if you put them in cool and allow them to heat with the water. I’ve always just put things in hot – hot jam into hot jars = hot tomatoes into hot jars – hot pickle juice over cucs in hot jars… (Have you canned before?)
Also, if it’s just tomato sauce you can “can” it, but if it’s a marinara sauce with lots of vegetables then you may or may NOT be able to can it strictly with a water bath.
Don’t worry, it’s really quite easy! Here’s my favorite website for canning
http://pickyourown.org/allaboutcanning.htm
Hope that helps!