I definitely wouldn't like it. I think it would actually freak me out a little, especially if I thought there was a chance someone might hear her. I mean, in most cases the models I work with are basically strangers to me, so if one started calling for help I might not be sure how serious she's being. I don't like the idea that the ticklee is so distressed that she actually feels the need to call for help. And even if I knew it was alright there's always the possibility that someone else (like my neighbors) might hear her, and I couldn't blame them for thinking something really wrong is going on.
Begging for mercy, though, that's a different story (as long as it's not too loud).