However, I am a little frustrated with some of our instructors. If you have never ever canned before you should absolutely follow a tested recipe to the letter the first few times, or any time you are trying a new technique. This is especially important for things like pickling wherein adding the appropriate amount of a specific acidic ingredient can be the difference between safe and dead. But this does not mean you cannot use your brain, which some of the instructors seem reluctant to allow us to do.
If you arm yourself with knowledge, there is no reason you cannot, for example, make your own fruit jam. If you are using fruits that are acidic enough to safely water bath can by themselves, then mixing them into your own personal concoction is not dangerous. Lowering the sugar content is not dangerous, but will affect the set of your jam and how long it holds in the refrigerator after you open it; if you know this, you can compensate for it. You do not need to use commercial lemon juice (which I find vile) if it is only being used as a flavoring agent and is not necessary to render the product safe for that preservation method. [Commercial lemon juice has a standardized acidity, therefore it is important to use it, or an equivalent citric acid solution, if the recipe specifies commercial lemon juice or you are doing something like making pickles. Again, use your brain.] The same goes for substituting vinegars in a recipe. If the vinegar you wish to use has the same, or higher, percentage of acid than what the recipe calls for, it is safe to make the switch. If it is a lower acidity or unknown, as is often the case in homemade vinegars, then you should not presume it is safe to use in place of what is called for in the recipe.
In the same vein, you should not presume that just because a recipe is printed in a published book it is automatically safe. There are books that you can make that assumption, such as the Ball® books and publications by Cooperative Extension offices, but there are a lot of books that have come out in the last few years which have recipes that have not been rigorously tested. I have some canning books that have recipes that I would not feel comfortable canning as is written.
God gave us a really big brain, let's use it. (And when in doubt, stick to a tested recipe!)