I have a concern about "strengthening" neck muscles w/TOS.

If you try to do so and those muscles are already over firing from nerves or spasms it won't do any good and may make things much worse..

I would only attempt to strengthen when all muscles are relaxed and not in spasms.

In the trigger point sticky thread there is more info about it-
If a muscle has trigger points/spasms - you cannot accomplish much until all of those are resolved..
http://neurotalk.psychcentral.com/thread125577.html

Passive stretching that allows the shoulders to relax & drop was very helpful for me - images here-
http://neurotalk.psychcentral.com/album.php?albumid=422