Yes, anyone who says "25hz" is on crack. They're probably confusing it with the (roughly) 25khz horizontal scan rate of the 512x384 mode. I'm actually somewhat skeptical that this is usually a root cause of their issue because they're obviously going to be trying to use a generic monitor with a DB15-to-HD15 adapter, and I wouldn't *think* there'd be many in circulation that have the sense lines set up for that mode because there were never very many monitors that supported it. (Pretty much be restricted to early generation "Multisyncs"; monitors that will happily sync with sub-31.5khz sources mostly died out by the early-1990s, and they were generally "premium" models.) Most of the monitor adapters I've seen that don't have mode selection switches are either "multisync" or locked at 640x480. That said...
So, regarding "setting up the adapter right", if you have one of the switchless ones you kind of need to figure out *what* it is. The "multisync" ones were targeted for later Macs (last-gen Quadras? and Power Macs) that have "Extended Sense Line" support, and I'm not positive LCs grok those properly; I'm not sure *what* video mode they'll default to with one attached. Re: the 640x480 ones, I think those come in both "real VGA" (640x480x60hz) and "Mac" (66.7hz) flavors; it was the LC that introduced the sense line combination for 60hz. Most CRT monitors will work fine with either unless you have a positively ancient fixed-frequency VGA monitor, like one that came with an IBM PS/2. Older LCD monitors will also generally work, but you *may* run into trouble with newer LCD monitors if the Mac is spitting out the 66.7hz refresh rate; in the mid-2000's they started getting lazier with the scaler boards and expecting the attached computer to honor DDC, rendering the monitors incapable of syncing with anything but a limited set of VESA modes. For these you'll pretty much have to have an adapter that *specifically* gives you the "real VGA" mode.
Here's another thing I'll just toss out there: LCs, in my experience, are one of the Macs that might give you a black screen (not initialize video at all) if their PRAM battery is dead.