The profile (temperature curve) is absolutely a function of both top and bottom zone settings as well as conveyor speed, blower speed, etc..
Certainly when you perform profiling you have to consider both top and bottom heaters when you make adjustments to generate the profile you want.
I can only think of one time that I deviated from top = bottom settings and I don't even remember why I did it. As far back as that was I suspect that it was a single sided, 63/37 board. Maybe it was warping due to an imbalance of copper and I was trying to reduce it, or I had issues with the board not wetting properly.....?
I only do limited profiling now (we're still trying to replace our last process engineer so I step in where needed) but haven't seen a need for this at all recently.
Frankly I would avoid it unless you come up with an issue that you feel can only be resolved with this, AND you can properly deal with the other problems it potentially presents.
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