Your answer is spot-on Dave.

But you can also use all eight bits in your TRIS statement, the upper bits of Ports that don't exist will simply be ignored.

This certainly works for multi-family PICs like the 16F876 who has a bigger brother (the 16F877) that does have those pins, and the internal archetecture is shared. If the Datasheet says the Register has eight bits (even though some are unused), you can still send eight bits to that Register, the unused ones will just be ignored.

The advantages are that is you ever migrate to the bigger PIC, your TRIS (and other Register) statements aren't going to throw you a curve-ball because you've accidentally left them undefined.

Some PICs (like the 18F2520) have a PortE.3 all on it's own. In this instance you can set the TRIS bit on it's own, or you can still send all eight (or just four) bits to the Port, however all but E.3 will be ignored.