A lot of it depends on weather, nightly events, crowd levels, etc.
Having the Park Hopper gives you maximum options depending on the situation.
A few tips:
1) Use it when crowd levels at MK get unmanageable at around lunch time.
2) Don't waste too much time travelling from park to park. Although you can, transportation can take up to an hour, especially if you can't use a monorail. I would recommend 2 parks in a day. I've done 3, but it does take extra time to jump around so much. I still think you can do MK to EPCOT and back to MK. I still consider that 2 parks in a day.
3) Use the monorail from MK to EPCOT and EPCOT to MK. It's fun and usually faster.
4) Use the boat or walk from EPCOT to HS. It's kind of far to walk, but there is a little used pathway specifically for walkers. It can be done in 15 minutes and I've done it multiple times.
5) You can get a little advanced at times. For example, see a Grand Floridian, Contemporary, or Polynesian bus right when you're leaving AK? I jump on those and then take the monorail to MK. It's fun, allows you to see other resorts, and potentially reduces transportation times. I've done this by taking a Beach Club bus and walking to EPCOT. I started doing this because it seemed the hotel buses came sooner than resort to resort buses.