While I don't use the traditional aquarium product root ferts, I have used Jobe's fertilizer spikes for close to 10 years now. However, I use them as spot fertilizers in that I only feed those plants which will benefit most from this sort of fertilizing- swords, crypts and a few others. The one thing I do is to put the spikes right down on the bottom glass under the plants I want to benefit. In order to avoid damaging the root systems I use a 10 inch long tweezers and I start from a bit outside the plant's root mass and push the ferts in at an angle. The object is to get them underneath the roots by the time the spike hits the bottom.
I am able to cut the spikes to size. This lets me use multiple smaller pieces for larger plants and right size pieces for others. One word of caution for anbody thinking of using Jobe's Spikes, they will really trash the water if the substrate is churned and the spike pieces are allowed to come out of the substrate and get into the water. So these are not a good substrate fert for folks who love to replant regularly. When it comes to using Jobe's, once in the substrate hang out a big "Do Not Disturb" sign.
As for placing Flourish or API tabs as a total substrate fert, I suggest you space about as suggested in the directions. If you have areas totally devoid of plants there is no need to put tabs there. However, bear in mind that plant root systems can become quite large so they will often extend out further than you would think.