I'm by no means an expert on the subject, but I've brewed many, many batches, and I know what worked better and what worked just marginally.
I promise you, if you follow the guidelines at http://www.mrmalty.com/calc/calc.html
you will brew better beer than if you just pour your package of liquid yeast into your wort.
So if your ultimate goal is to brew beer (and that's a perfectly admirable goal), just pour your package of liquid yeast into your batch and you will accomplish your goal.
If your ultimate goal is to control all the factors you reasonably can, to brew the best beer you can, follow that link and take the advice given therein.