I have encountered a similar situation w/r to placing sums only in footers.

What I did instead was to generate in T-SQL a table variable which is first populated by the raw data then another T-SQL statement inserts summed up records. I have added an extra column which flags each record as Raw or Sum and use the SSRS's Conditional Row Visibility to print a subtotal record in a group of the raw records (also filtered as per the Raw flag).