First off, sessioning with any quality Domme will involve pre session negotiation. Subs should be clear about listing both their interests and their limits. Most Dommes also list their interests/offerings are and are usually very clear their "no go" areas on their web page. If GS isn't on either party's limit list then I'd say it's always a possibility although I think the likelihood depends on the type and style of session the sub has indicated he is looking for.

A great session can involve a "surprise" and often benefit from it as long as it's from the list of "might happen" stuff.