I'm sure some a lot you out there will disagree, however this my blog and my opinion, but feel free to argue if you wish.
I believe that a good Dom will insist that you should have a safe word. A word that, if spoken, halts any activity taking place immediately and without repercussion.
If someone tries to yell you safewords are not necessary or that you'll get punished if you use it, that suggests to me that he or she does not have your best interests at heart.
In my years of experience I've always had that in place, but my Subs have never had cause to use it. A Dom who knows their sub should able to read the tell tale signs, know where the limits are, if and when they can be stretched a little, and most importantly when enough is enough.
As in all relationships, but even more so in a DS relationship, trust and faith in your partner are paramount. It takes time to know one another, to build that faith. To make that connection.
And then we come to aftercare... but I'll save that for another little rant.