There are a lot of conflicting observations being mentioned. I totally understand not bringing loose articles on rides- I'm all for safety. BUT, when you talk about checking pockets- that's going way too far!
I'm a Disney veteran and am always amazed at how you can bring a backpack on almost every ride. People have phones/cameras in their hands on rides like Dumbo, TOT, etc... and the ride operators don't say a word! We have also been to Six Flags, Hershey Park, etc... and gone on upside down flipping coasters with wallets, phones, keys, in pockets and never had a problem with losing anything.
You can not start checking pockets- it would waste too much time and be an invasion of privacy!