When your design requirements add up to an odd number of layers, it almost always makes sense to add another layer.
There are several good reasons for this;
An extra ground plane can be used to provide a more robust signal return.
Board construction is more likely to be symmetrical, reducing the likelyhood of excessive warp and twist.
The difference between 3 - 4, 5 - 6, 7 - 8 layers, etc. will not increase the cost of your order.
You could add one more signal or power layer in order to reduce routing density. 5/23/2011 2:32:37 PM