It is always better and emptier to be in park at 6:30 or stay and scream before then. Doing that you can probably do at least 5 houses with little to no wait. I think around 8 is when the waits really increase and usually one house jumps up to 60 minutes during that time followed by others. I think the lines die down a little around midnight and even at 2 AM or much longer than they are at 6:30, however between that time there should be 2 or more houses with less than 20 minutes wait. If you use the app., the times posted at the houses are the same as the ones on the official universal app.
Personally I think the best and scariest how is Scarecrow; followed closely by Dead Waters (Best Scenery) and American Horror (especially if you have seen any of the seasons; however I think Ash, the Hive, the Fallen, and the Shining are all right there too, with Saw being the worse and way behind the others.
Have fun, but if you can get there earlier, if not utilize the app. to make the most out of your time.