Visuals are fine. I find that powder check dies are easier to use, and it's a lot easier to spot an over/under charge than eyeballing a single case, especially with deep cases like .38 and .357. Having said that, the important thing is to identify a large deviation. Of course the thing with a powder check die, unless it is a lock out or audible warning style, is that you must look at it every time you pull the handle on a charged case. The same is true with a visual inspection. IMO EVERY round must be checked for a gross deviation. Checking 25% of them, just reduces the chances of a squib/overload by 25%. Spot weighings are necessary, but they are designed to make sure your measure isn't drifting.
Do you have room for a powder check die? FWIW, I use a powder cop die for my deep cases, and a visual for my shallow auto cases. I only have 4 stations on my turret, and prefer to seat and crimp separately when possible. I feel that the visual I get with, say 9mm, cases is sufficient to identify problem charges. With deep cases and charges being a small percentage of case volume, I use the powder check, even if it means having to size/decap in a separate operation. Bottleneck rifle calibers have every charge weighed and then visually inspected prior to seating a bullet. Of course, they are low production numbers for rifle calibers, and I drop powder for them using a stand alone measure vs a die mounted for pistol calibers.