To minimize movement of roofing panels a combination of fixed cleats and expansion cleats are required. The top part of an expansion cleat can slide back and forth in the base allowing the metal to expand and contract.
Low slope (or low pitch) roofs generally use longer panels than a steep roof. The longer the panel - the more linear expansion can be expected (see chart of expansion for various materials). To reduce the effect this has on the ridge or eave the fixed cleats should be placed in the center with expansion cleats on either side. This will split the expansion at each end in half.
Since a medium pitch roof will usually be shorter there is less material expansion to deal with. The fixed cleats can be placed closer to the ridge - approx. 2/3 of the way up.
On steep roofs the fixed cleats can be placed at the ridge
with the expansion cleats towards the eave.