i've attended since 2000 and will answer your question based on last years
hhn.
i always buy the ep for each night i attend.
i spend too much money to waste my time in long lines.
going the 2nd weekend to hhn last year, the ep saved me loads of wait time.
there was only 2 houses where my wait was 25 minutes. the shortest wait was about 5 minutes.
i do the stay and scream area and get a head start on the houses. there are usually 2 maybe 3 houses that open around 6. general admission is let in at 6:30.
i usually start using my ep around 9 in the evening. if i can get in the house in the regular line within 20 minutes, i save the ep for that house for later. i do each house twice when i am there.
lines are not bad in the beginning of the evening.
on peak nights, i have found the crowds to lighten up around 12:30 and non peak around 11ish.
the only rides i use my ep for are jaws and the mummy. i do the rides in the daytime and the houses and scarezones in the evening.
the ep is generally only accepted for the first show of Bill and Ted's adventure. the first show is usually the lightest crowd wise.
you will have fun!
try to get an early start and you won't have to be too concerned about long lines in the early part of the evening.