Currently, at a line item level, it is possible to set a specific summary option for Time. It would be beneficial in terms of space and performance to be able to do so for each list a line item applies to.
For example, we often have a list called "scenario" with a top level item so that we can create scenarii by "copy branch". Even if it does not make sense to sum by scenarii, we often have to set the summary option to "sum" in order to sum up by the other lists the line item applies to.