It depends on where I am and how tight I expect accommodation to be. On my last three overseas trips, I've booked well in advance for the night after I arrive and the night before I leave. I don't want to go scrounging for cheap accommodation in a strange city/country when I'm jet-lagged and sleep deprived after a long flight, and I want to know how far it is to the airport and how I'm going to get there when I've got to catch a flight home and get to work the next day. The rest of the time I play it by ear, sometimes I book ahead and sometimes I just show up. I've had some close calls where I thought I wouldn't have to book ahead and I ended up wandering the streets in a drenching rain trying to find accommodation (that's when I decided to bring camping gear on the next tour).