It depends which rides you want to do and how many times you want to ride them.

There are five (sometimes four if Splash is off the list) Donald rides at MK. If you want to ride all of them once, you can use three Donald credits and use your ticket for two additional FPs to get the other two rides - easy enough to do. But if you want to ride them more than once, you'll get a lot more done by going early. At DHS, if you want to ride TSM more than once and also ride ToT and RnR, it would be good to arrive early. I'm not sure ToT ever runs out of FPs, but TSM runs out by early afternoon and RnR sometimes runs out. Many people use the Donald credits for three rides on TSM, then they use park tickets for ToT and RnR - totally doable if you arrive by lunch or early afternoon.
I would think you could definitely sleep late for Epcot and AK.
The GAD FPs give you a lot of flexibility and help you avoid criss-crossing the parks to get FPs. But going early is never a mistake - you can just get more done.
We are planning to use a GAD voucher for FPs at DHS and we are intentionally NOT arriving for rope drop. We don't want to deal with the morning stampede. But we will be there within 30 minutes of opening. We will be there for RD at MK - my kids like to ride the MK rides over and over again. The GAD FPs are just a bonus to our regular touring plan.
Edited to add: If you want to see the Fairies in Toontown, it is still a good idea to arrive early so you can make Toontown rope drop at 9:45.