I tend to do OLCI and print the boarding pass at the airport (mainly because I'm sad and keep the boarding pass stubs as little momentos of my travel!).

However, all of the above posts assume that you are travelling solo - if you are travelling as a couple or family then OLCI stops you getting separated (unless you want to be ). My sister ended up having to sit in a completely different part of the J cabin to her husband flying CPT to LHR as she checked in when she arrived at the airport...