In addition to Shaun's point i would note that you can always re-upload data so if you are missing information or make a mistake you can always fix it later.
Removing dormant contacts is also an issue that most clients run in to down the line when trying to reduce the size of your database. So if you can filter out contacts you know that you cannot market to or don't have enough information yet to segment on, now would be a good time to keep them out of Eloqua to keep the database relevant.