Are you expected to tip after a private viewing session when no extras are provided?  I don't know the scene so I don't know what I should do.  Thanks for any advice.

Can you give us some more info ?

Private viewing is usually non FS nude touching with happy ending.  Halogen type agency.

I think it's generally a good idea to tip regardless, after all they are still providing you with some kind of service. Not as much as a FS provider but something nonetheless.  Of course if you had a bad experience and/or you don't plan on seeing that provider or agency again that's up to you.  

I've found that private viewings vary. Some I've used do end up w/FS.  Other's as you've indicated just include a nude massage w/happy ending. Then there are some that after you've frequented them for a while allow the FS. This definitely falls under the YMMV category.