Entertainment guru Mark Evanier posted on his blog an excellent essay on deadlines ( http://www.newsfromme.com/archives/2010_08_15.html#019381 ) and an equally excellent follow-up (http://www.newsfromme.com/archives/2010_08_21.html#019401). They contain several solid recommendations -- backed up by the sort of anecdotes a professional storyteller delivers best -- but the central message is that no matter how brilliant your work is, if you can't turn it in by the assigned or promised date, you're unlikely to get the next job.
This advice applies not only to creative work, but the perhaps-less-creative field of Law. In law, blowing deadlines can have consequences far more serious than losing future work (although that can happen too).
It's also important to remember that reliability doesn't just mean meeting the deadline. It also means turning in good work at the deadline. Two deadline risks are presenting ineffective work product at the last minute; and what I call polishing the fire engine while the alarm is ringing -- revising and revising a project to make it as good as it can be, which can ironically result in more errors since the project can't be checked before it's turned in.
All this means that a person should learn that person's abilities and realistically assess how long a project will take -- which is what Evanier recommends in his second post. That's what enables people to turn in good work on deadline. That's one of the hallmarks of a professional.