Do you always use the same formula? Do you ever mix and match?

Of course there's no way of knowing how things will go until our LO is here, but right now we're planning on both BFing & supplementing regularly with formula. I've signed up with a couple different formula companies, and got full-size samples in the mail. Right now I have two different Similac formulas & two different Enfamil formulas. I would hate to waste all of this free formula, but I'm not sure if it's okay/normal to mix and match. What do you do?!