With the ParametersSummary, it is indeed currently not possible to manipulate the decimals.

Depending on your use case, some other options are to:

• Use the param placeholder(s) in combination with the calculation placeholder(s) (sum, subtract...) and wrap the entire thing in a regex placeholder that does the rounding.
• Executing a script that outputs the result of your parameter summary to a variable, already displayed correctly.

Both options are kind of dirty though, so it might be best to suggest this as a feature.