from what I understand, the substrate does not really interact with the water column in terms of nutrient exchange. So dosing the water will not really help the substrate, and using root tabs shouldn't ideally add nutrients to your water.
Flourite and other plant substrates are already enriched in iron, anyway! I guess if the substrate is old, we're talking years old, and your plants' roots have already deprived all the nutrients in the substrate (if you notice roots are having problems), then you can replenish them with root tabs buried deep under the substrate.
I mis-used root tabs and put them too shallow. So the next day, my water was full of ammonia and nitrite, yikes!
Some advanced aquatic gardeners (djlen comes to mind) don't even bother with substrate fertilization. Len says that religious water dosing does him just fine. I'm going to go with some plant substrate (laterite) though because my plants are growing roots and are popping out of the substrate all the time.