I've sold probably 6-8 steel tanks, as well as buying twice that many. I've never personally opened a tank before buying, and most of the time they're way out of hydro when I buy them. I am certified to VIP tanks, and I have a light and probe (and a gauge, and a 2x miror, and...). But I've never done it. And I've never gotten a bad tank. YMMV, of course, but I've found that if the outside is in obviously good shape I haven't had a problem with the inside.
The tanks I'm buying, though, are in Michigan. They've only seen freshwater and many of them are obviously infrequently used. Maybe you'd have to worry about a tank that was filled at a place with poor filtration, but generally the quality of gas we get here is very good. And so many of these tanks have literally been untouched in someone's basement for 10+ years; you know, slightly after the last hydro was done!
I've had people whine and complain about the tanks I've sold being out of hydro. I've told them that I'd happily add $40 to the price to hydro and viz the tanks (the going rate here), but that they'd have to pay for the hydro up front and I'd be happy to make sure they got perfectly new-hydroed tanks. No one's ever done that. And no one's ever asked to look inside, either. If someone insisted on cracking open a tank, I would be *happy* to, under one condition: they pay for the tank. If the tank fails viz standards, I will refund their money. In other words, I'm not screwing around with it unless it's a done deal, and you want to be safe.
Now that I write this, maybe I'll offer a peek in the tank right at the beginning, to put people at ease. Of course, there's usually *gas* in the tank... And I hate cracking steels, because of things like humidity and flash rust. Honestly, if you're buying used steel SCUBA tanks in Michigan, you already know what you're doing, and you're probably just trying to use the hydro/viz thing to beat me down on price...