In reply to Otis:
Depends on the rules!
In the BMC leading ladder, clipping the lower-offs.
In the Quay Climbing Centre's 'leading ladder', clipping the lower-offs AND holding the top hold in control.
In the Quay summer boulder league, top hold in control (boulder rooms).
In the Barn boulder competitions, topping hold (artificial boulder).
And if you aren't doing a thing with rules? Then the rules are up to you...
Obviously outside there aren't any 'holds' so clipping the chains seems obvious, as does topping out on a boulder problem that tops out!