When I'm in the top role I encourage my subs to use Mercy as in "Please Mistress, mercy on my butt" or "Mistress, may I please have mercy on my wrists, the rope is starting to cut off circulation". I just find using it keeps the feeling of the scene as well as enabling open communication between myself and the sub. In casual play quite often subs will have their own safeword which is then my duty to heed when they say it, usually it's red, once guy I know had a safeword of ambulance
One partner I had insisted on potato being used as a safeword - I was bottoming at the time and was thankful I didn't need to use it, I would have cracked up.
My own personally opinion is that the closer and more intimate the relationship the less of a need for safewords there is. I know a couple of people in relationships like that where they don't have a safeword but the dominant knows the submissive *very* well and the sub trusts the dom not to harm them. It takes while to build up that kind of trust though.