Writing For Orchestra: Basses With C-Extensions?

Good afternoon, everyone. 

I have recently begun composing a work for full orchestra, and am moderately versed in orchestration with regards to the capabilities of each instrument and their players. However, there are some things that even an orchestration book can't tell me. 

The work is in D minor, and I like very bass-y things, so I'm using the basses and cellos in parallel octaves for an introduction, and as a home base (get it?) when things get quiet but I need to keep it moving. However, I am scoring it with the assumption that whatever orchestra in my area may consider playing it (don't worry about that part) will have bassists who own C-string extensions so as to tune their E-string to a low D. Is this a fair assumption, or will I need to reorchestrate? 

Thank you for your time, and I wish you all a fantastic day!

- Alpha Spartan A

If you are shooting for performance, pin down the ensembles, track down the bassists, and ask them first.

It is quite common for orchestra double bass players to have low C strings.