Example: I create task and initially estimate it for 2 weeks. After few days I start working on that issue but 2 weeks is too long for single issue, so I divide it to multiple subtasks. If I estimate subtasks subsequently project's estimate will grow artificially because I estimate subtasks having estimated parent task.
End of example

In my view after dividing task into subtasks original estimate of parent task should be 0 and should be calculated from all relevant subtasks.
Other solution would be not to include estimations from subtasks into the project/release estimation sum. This way after logging work for subtask remaining estimation for parent task should be decremented accordingly.