Can you mix old potting soil with new potting soil?

Can you actually blend old and new potting soil together? I've heard some gardeners swear by this method, claiming it saves money and provides a good balance of nutrients. But on the other hand, I've also heard that using old soil can introduce harmful pathogens and pests into your plants. What's your take on this? Is it SAFE to mix old and new potting soil, or is it better to stick with fresh soil exclusively? And if it is safe to mix, what's the ideal ratio of old to new soil?