Number one, you have to be a Disney Hotel guest. Extra Magic Hours are a perk for Disney Hotel guests.
Number two, you should do it when you've got park hoppers. If you got to Extra Magic Hours, the park fills up, you want to jump to another park that's a good thing.
Number three, if you don't have park hoppers you do it on a light to moderate crowd days. When it gets busier than moderate crowd days the Extra Magic Hours park will be, especially the morning Extra Magic Hours this doesn't really apply to the evening Extra Magic Hours, the morning Extra Magic Hour park will be the busier park typically. Magic Kingdom sometimes skews that off a little bit. Magic Kingdom's always busy but the other parks you might want to avoid those parks on morning Extra Magic Hours days.
Number four, evening Extra Magic Hours anytime you can. Evening Extra Magic Hours are great. Most people will leave the parks after the fireworks, after the parades and evening Extra Magic Hours are only available at the Magic Kingdom and Epcot. So after those fireworks, after those parades there's nobody there. Evening Extra Magic Hours, if you can stay up that late, sometimes it's 2:00, 3:00, 4:00 in the morning, those are really cool.
Dad's Bottom Line
So Dad's bottom line, Extra Magic Hours are KEWL. Use them when you can.