I do think the single most important thing is having a good touring plan and getting there at rope drop; more so than whether or not you avoid or attend a park with EMH.
Like most things, whether to attend or avoid EMH depends on many factors (crowds, weather, what day of the week it is, rides you want to do, are you a morning person, are there events/parties that night, plans for the rest of the day, plans for the rest of your trip). I never automatically dismiss EMH, and never mindlessly schedule EMH. Like PP says, the most important plan is to be there for rope drop, have your FP+ mapped out, and know what you're doing.
On our first trips, during very low crowd seasons, we always hit every EMH, because park hours were short (7pm closings), and the parks never got so crowded during the day, we could always get rides in. Nowadays the "low" crowd seasons aren't as low as they used to be, we avoid HS and Epcot EMH like the plague. When we've been to WDW during more busy weeks, we've selectively gone to morning EMH with the plan to park hop, or started at one park and then hopped to MK for late EMH. My parents, on the other hand, always go to every morning EMH. They're up at 5am anyway, and they don't care about doing the roller coasters, so they don't care how long the lines are in the afternoon.
