I guess part of my question would be, how often do you do the extension week... If it is likely do the final week, then I'd say adding it as part of the contract term is understandable. I would at least like to know that's the case. If I'm taking a regional theatre gig, I don't expect the theatre to close the show earlier than the contract term. Commercial theatre, long running shows - totally different, but on a LORT? I'd feel blind-sided.
I would also think that it would change the energy on the production... I know that if I'm working on a show and it extends, there is a sense of "Wow, we're doing really well, people like us, isn't that great!" which creates a positive mood. When the show doesn't extend, it's not really that big of a deal (it wasn't expected anyway, poor marketing, a hard sell of a show, lots of competition for entertainment dollars, etc). But if a show closes early, that seems a little more crushing on the spirit. for you who works there all the time, seeing how these things really go down, it may not be an issue; but for someone who comes in and does 1 or 2 shows a season, or just someone new to your company, it could leave a negative feeling...
Just my 2 cents.