When creating a loft is it better to have the guidelines in the middle or on the sides(cube)?
If you're creating the guide curve in the center of the profiles, use the centerline parameters option. This option allows you to have the profiles remain symmetric about the center of the profiles. If you use guide curves that pierce the sides of the profiles the profile will only fully match the guide curve at that side. Use the guide curves option when you need more control over the sides of the profile as opposed to the overall path.
It also depends on what you’re after. A chain’s slip hook is a perfect example of a centerline loft since making the side rails would be quite difficult. However a twisting belt (depending on the rotation) will only work with side rails since a centerline doesn’t control the rotation of the profile.