It's usually on a character or series basis. Was the series a big hit? is it a generally well-liked character?
Price prediction has to do with understanding the demand. If it's a figure you KNOW people will buy in droves, it's usually better to PO. There will be low chances that any will be left by the time people post their own pictures, and the ones being re-sold will be more expensive (to make use of the demand). A figure with more questionable sales is fine to hold off on for the exact opposite reasons.
When it comes to figures around the middle, it's really more of an educated guess than anything else. I personally just say "screw it" and PO to be on the safe side.