We just did this, saving FP for MK. It worked very well. If you arrive at RD, you can do most of the headliners at AK without a problem. We saved our FP for Splash Mountain, SDMT, and BTMRR at MK that night. All of those had posted waits of 40 - 70 minutes.
Single rider line wasn't open at EE when we were there (a Monday in July), but the line was only a posted 20 minute wait at 10 am.
MK just tends to have the longer lines, and if you want to do any of the biggies at MK, you will almost always save more time by using your FP there than at AK.
If you really have a lot of planning time now, you could check out
MDE and see general wait times for the rides you would consider getting a FP for at the times you would consider being there, and compare that with the wait times at the rides at MK you would consider fastpassing there.
Like the previous poster, we did AK twice in the morning and were able to get in everything without long waits. We saved our afternoon/evening fastpasses for other parks.