Whilst this is still good advice, United seem to have removed the "cancel before departure" clause from at least most of their refundable tickets, which means that (as findark has said above) there's technically no need to cancel the trip before departure - the fare will still be fully refundable after departure if it's unused.

Of course, as always it depends on exactly what is written in the fare rules, which can vary depending on a number of factors (including point-of-sale), so it would still be good advice to make sure it's cancelled before the departure time.

The no value after departure/no-show rules still exist for non-refundable tickets.