I think the truth is somewhere in between here. Brass that isexcessively long can be dangerous. That's a fact. But just how often you need to trim your cases is a whole different subject. There are scads of variables at play on this subject. First and foremost is the pressure of your loads. Probably next in importance is the cartridge...then your particular chamber and rifle.......followed by the type of brass you are using. And there are probably a few I'm leaving out.

Loading really high pressure loads causes brass to flow/stretch. Backing off a few grains with your load will often greatly reduce any neck growth. Ex. Supposedly the 220 Swift is terrible about lengthening cases. I had one for several years and found my cases hardly ever needed trimming. But I loaded a few grains under max pressure. One of the worst cases I've encountered for necks stretching is the .243.

No two reloaders treat this issue exactly the same but I've found the subject greatly overblown in most instances.

The best thing I've found for case trimming is the RCBS File Trim Die. They are cheap and fast and easy to use. I've got a good crank trimmer and never use the SOB if I can avoid it. When I do decide to use the cursed thing, I generally set it to trim my brass about .002 shorter than factory just to be sure it's a long time before I have to play this game again.

Rather than rush out and spend a bunch of money, I would do some case length checking and find out IF and just how bad a problem you really have. Then and only then would I worry about it. I've been loading for over 50 years and never had a case length problem...and I admit I'm a little sloppy about the subject. But I think this should illustrate just how big a deal it really is.