Laying out plans on left-over stock

I'm a long-time viewer but a first-time poster on these forums. I have a question to might sound very dumb regarding how you start a new project with a set of plans: Are there any rules-of-thumb or guidelines that you guys use when deciding how to lay out the parts on existing wood stock, and/or how do you decide if you need to buy new stock?

Here's where I'm coming from, I did my first 'big boy' project about 7 years ago when I built an oak bed frame for my son. Bought plans online, when...

Laying out plans on left-over stock