This machine runs NT 4 Service Pack 4 already. Ran them both before and after the update. Meanwhile, I decided to email someone who had a real reason for an opinion. To this end, I asked Jamie Hanrahan of Kernel Mode Systems his opinion. But this is something I'm only reasonably certain of and that I've been meaning to nail down.
Editors are important.
In this case it's not so much dangerous as time consuming. In both cases, the information is correct, but incomplete. You should only consider editing the default value if you are running computers with direct-mapped L2 caches. Pentium II and later processors do not have direct-mapped L2 caches. However, defeating the operating system's default behavior and setting the value of the SecondLevelDataCache to 256 KB rather than 2 MB when the computer has a 2 MB L2 cache reduces performance slightly.