Never into a sewage line going to a septic tank or other on-site sewage disposal system.
Almost all cities prohibit running gutter water or ground water or rain water into the sanitary sewer. Some cities let you drain to a storm sewer which is a separate system from the sanitary sewer.
Acceptable places to drain underground gutter collector piping:
1. Into a pit where a sump pump sends it away from the house,
2. Into a dedicated leach field (which can be constructed in the same manner as a French drain, in the same manner as a septic tank leach field, or any method in between),
3. Terrain permitting, a down sloping undrground pipe that eventually reaches the surface.
Do you get coverage you can count on? From the bag of fertilizer? From your lawn sprinklers?