I find this one of those things that has a fine line between pleasure and pain. For me it's got to be built up slowly - some regulars have got this off to a fine art. Starting by just rubbing, or scraping with a fingernail through my shirt while we start DFK, building up to a bit more with the shirt off, and then a bit of licking and sucking. Finally some nibbling and harder sucking.
I agree though, that a hard unexpected tweak can kill the moment, so it's a fine balance for me, and WGs that get it right have a valuable skill in my eyes - or they're just lucky!