It does depend on the park and when you are going but in general I find it to be during the afternoon. Mornings and evenings don't seem to be as bad as mid-day. This is why we often take a break and go back to the resorts during the middle of the day.
This simple truth is why I think that using your 3 FP+ reservations as early as possible so that you can start getting additional ones as soon as possible may be self defeating. You might be able to get more FPs over the course of the day, but you probably won't be able to ride more of the top attractions, and may very well end up doing less.
Unless and until crowd patterns change significantly, I think you should use your 3 advance reservations in the early afternoon if you plan to stay in the park all day.
If you plan to take a break in the middle of the day and hop to a different park, I would make my reservations at the second park and do as much as possible in the first park by arriving at RD and riding until lines get too long for your tastes.
If you plan to take a break and return to the same park, you could consider using one or two of the FPs before the break and the balance after.
I think this general approach lets you use your FPs for the things you want at the times you want instead of taking your chances on what happens to be available after you have used your initial 3.
Just my opinion. But, the main basis for it is that even the reports of longer standby lines at some attractions seem to focus on posted wait times in the afternoon, not in the first 2-3 hours after the park opens. It's also important to keep in mind that, unless someone actually waits in line, those posted wait times may be overstated, and maybe by a significant amount. We have often found, especially in the first couple of hours after opening, that rides will have posted wait times of 20-30 minutes when they are, in fact, virtual walk ons.