That was the general rule of thumb before the extra FP+'s. Now, it depends on the answers to 2 question.
1) How effectively can you tour the each park without FP+?
2) How easy do you think it is going to be to book extra FP+ at the second park?
For example, if you are planning on RDing DAK and Hopping to DHS, I would probably use the strategy you said; do DAK with all standby and use your prebooked FP+ for DHS. DAK is still fairly easy to do in the mornings without FP+ and prebooking FP+ at DHS guarantees you TSMM or RnR, ToT, and one other Tier 2 ride.
On the other hand, if I was starting the day at DHS then hopping to the Magic Kingdom. Assuming 7DMT and A&E are not on your "Must Do" for that day, I would probably swap thing. Prebook my FP+ at DHS, so far at least, there seems to be good availability of FP+ for solid Non-Headliners later in the day. Use your Extra FP+ for those rides early evening, catching a headliner if it comes along, then if the parks open late enough, and you have enough energy, ride the headliners after the fireworks.