I always do this, just to get it out of the way and feel that I'm "on" the flight and "in" a seat. For most of my flights these days it's possible to print out the boarding pass at the airport (LHR, bmi) by entering the booking code at a terminal rather than going to a desk. In the UK for international flights, unless you've got baggage to check, this allows you to get to the airport an hour or so later.