by all means, use what you have around the home...I'm using an old poly/cotton blend sheet set for my current project because it had a tear at one end of the flat sheet and the elastic was worn out on the fitted (and my toddler wrote on a section in permanant marker). It's MUCH less expensive to purchase older sheet sets from a rummage sale or re-sell it shop than to buy yards and yards of white fabric for a big project. I have also used sheets for backings because they are solid and I don't have to piece it.
Your idea to use scraps for practice is fantastic! When we start out, a little group of ladies donate fabric to the new girl so she can get a head start, but without that help, I'm sure I would have been digging in the hand me downs. I also save my buttons on worn out clothes that can't be passed down, remove trimmings, and keep favorite pieces before recycling the rest to a mom with 2 kids younger than mine.