It varies by company and manufacturer. There is no real wrong answer ( within reason ) as long as there is enough meat on the top and bottom so the grommet doesn't pull out and you can still get your S-Hook in .
Most real estate companies already have signs, so you can match the spacing.
If not, use the most common hole spacing for your panel size in your area.
For wider panels like a 36" wide panel, the bottom hole spacing for the riders may be a little closer than you would think it would be,
so the customer can fit a more common 24" rider on the bottom.
In dealing with RE signs daily, we ALWAYS grommet the holes and recommend our customers do if they have the signs made elsewhere.
Even the steel panels will tear out the mounting holes if they are not grommetted.
As for the 5th hole for a wind chain to the post, the only signs I have seen with these are the very tall ones like an 18x30 or 36 sign.
They do help on the taller panels but don't do much for an 18x24 or 24x24 panel.