Hi Jared - There are two things to look at:
How many assignees are on the task
What is your setting for Setup -> Project Preferences -> Projects -> Timelines -> "When multiple users are assigned to a task, use the schedule of the..."
If only one employee is assigned, it does change the project schedule and push the task's planned completion date. If you have multiple assignees, it will then make the decision based on the setting I mentioned. You can have it look at the Task/Issue Owner or it can look at the project's schedule. We have it look at the project's schedule so the planned date would not get pushed even if one of the assignees is out the whole week. Anthony Imgrund FCB