am I being unreasonably paranoid?
No, but don't assume you won't be able to test your mix;
give your dive op. the benefit of the doubt and ask about it. I've seen a range of practices; on my live-aboard trips there was always a log and divers were expected to not just personally analyze their tanks, but log the results, %02 and MOD. And I've been on a day boat where the subject wasn't brought up. In situations where most trust the stated tank mixes, the boat might not automatically get the analyzer out and announce it.
On Scuba Board, a few years back it seemed there was a powerful domineering sentiment that you must personally analyze every single nitrox tank you ever dive. The only possible exceptions were when you watched the dive op. employee analyze your tank in your presence and the tank didn't exit your custody, or you had an immediate blood relative (e.g.: spouse) with a strong relationship whose judgment you trusted (didn't get the sense that was considered on par with testing it yourself).
In more recent times, when the subject comes up, there's still a pretty strong 'pro-testing' sentiment (understandable, you may read anecdotes supporting it), but it doesn't seem quite as severe, and most divers still don't test for CO. People dive tank after tank of air untested, but balk at untested nitrox tanks.
I wish we had a reliable estimate of the real world risk of a 'bad tank' where the O2 mix (or CO contamination) would be off enough to jeopardize health.
Richard.