Understanding Arrangement of second order sections
Different orderings of second-order blocks have different behavior with respect to quantization noise and overflow. Usually it is best to avoid very peaky transfer functions in second order blocks, because these peaks may cause overflow and quantization noise amplification. This is why it is common to pair poles and zeros that are close to each other, so the zeros cancel part of the peaks caused by the poles. The pole-zero pairing determines the second-order sections, but not their order. Concerning the order, there is always a best choice minimizing the quantization noise, but this order needs to be determined individually for the filter under consideration. However, there are two types of ordering that work well in most cases. Depending on the way scaling is performed, either ordering the sections with increasing or decreasing pole radii is best. In Matlab, these two common ways of ordering are referred to as 'up' and 'down', respectively. Have a look at this document for more detailed information.