Yes; decide if you want to go round trip from Seattle or Vancouver (or San Francisco) OR all the way across the Bay of Alaska, either to or from Anchorage/Whittier. If you are going to do a land tour also, you will need to go to Anchorage/Whittier.
In spite of what your friend said about Northbound cruises, I have to agree with Cruise Fanatic: I would much rather do the land tour first and then relax on the cruise. And as Dave pointed out, you will have more air choices from Seattle or Vancouver AND a shorter flight.
But whatever you decide, I know you will have an absolutely fantastic vacation. Alaska is just incredible.