I guess that, strictly speaking, if all the bars are showing as locked on the control panel and that's a pretty failsafe system, then visual checks probably are sufficient. Obviously in places like Europa they only have the visual checks, but they would always be pushing down on bars if they thought they were raised too high - especially on Silver Star. Despite the seemingly relaxed attitude towards restraint checks, they always appeared very thorough in their visual checks.
I mean, if you think about it - on Nemesis and Oblivion, they only really tap the belt to make sure it's in. And if like Titler says, they serve no purpose except a psychological comfort to guests, then what's even the point of doing that if they're not checking the actual restraint?