I always keep close to the right. If the truck does brake hard I can squirt around him, and I can also see if the bike lane is clear...wouldn't do to enter the lane blindly at 40-50 mph and collide with someone/something.
Also, the still air behind a truck is HUGE. You don't have to hug the bumper. Depending on speed, you can follow at two or three or five bike lengths and still push zero air.
Not saying it's safe by any means, and it's a good way to piss a truck driver off. Apart from that, it's fun, good workout, and a crowd pleaser. I especially love the looks I get from kids in the back of cars going past.
BL