First off, there’s some confusion about what standover is for. Most people think standover clearance is a safety measure to keep you from hitting your crotch on the toptube. This is actually not true – you will *never* hit your crotch on the toptube in a riding situation, unless your solution to a poor choice of line is to take both feet out of the pedals and lurch slowly forward off the saddle. In reality, a crash in which the bike stops suddenly and you don’t go over the bars will invariably result in smashing yourself into the STEM, not the toptube.
I have been riding bikes for a LONG time, and I have crashed them every possible way, and I have never once hit my crotch on the toptube. Not once.
In actuality, the situation (on a mountain bike) where standover is helpful is when you’re getting on or off of the bike in a tricky spot (such as an offcamber hill). You want to be able to get your foot down while keeping the other foot clipped in (or keep a foot down while clipping in) and if the toptube is too high, this is very hard to do.
That’s a pretty important thing to be able to do, so standover does matter. Just not for the reason most folks think it does.
The second point I want to make is that there isn’t a standard way of measuring standover. If you throw a leg over your favorite frame and stand over it, you’ll notice that you’re standing just in front of the saddle, maybe even with the nose of the saddle touching your back. Your crotch is much closer to the seat tube than the head tube. Back in the day, this wasn’t relevant, because toptubes were all pretty much level from the front to the back. But I do a lot of bikes with 20+ degrees of toptube slope – so the standover changes depending on where you measure it.
Unfortunately it’s hard to predict *exactly* where you’ll stand over the bike, so I measure standover at the midpoint of the toptube between the seat tube and head tube. But many manufacturers measure at other points, or in some cases, simply make up a standover number (at least as far as I can tell). The only way to check standover on a bike, really, is to either do the trigonometry yourself or else measure the bike in person. Standover numbers on websites are wrong at least half the time, in my experience.
How much do you need? That depends on your preferences with regard to tricky mounts and dismounts. Some folks really don’t need any standover. Others need 3 or 4 inches to feel comfortable getting on and off. Generally, the more experienced a rider you are, the less you’ll need. And on road bikes, you really don’t need any at all – there’s no such thing as a tricky mounting situation on a road bike, at least if you have any idea at all what you’re doing.