oh boy, you've stepped in it now
The job of the spring is to support the vehicle at the desired ride height. If it's doing that, it's doing it's job and there's no point in putting another thought into it.
Spring rate needs to fall between two limits. The minimum limit is set by the compressed height of the spring because low rate->longer wire->more coils->taller height when stacked solid - you don't want to lose travel because your spring turns solid metal before anything else makes contact. The maximum limit set by the free length of the spring because high rate->less difference between free length and ride height - free length needs to fill the space between the spring perches at full droop because you don't want to lose travel because your spring is loose before the rest of your suspension tops out.
Once you put those boundaries around it, you don't have a wide enough range of spring rates to make a significant impact to the way it rides.
In regards to on-road performance, it does get more complex than that. (For off-road, particularly rock crawling, generally softer is better to a limit.)
The spring must also be able to support varying loads without excessive sag. The stiffer the spring, the less of an impact loading has on it. But at the same time, it must also fill the parameters you gave.
As a general rule of thumb, the stiffer the spring, the harsher the ride quality. That's why Super Dutys with the plow prep package warn of loss of ride quality. That is a sacrifice made to accommodate a wider range of loads (i.e., with/without a plow).
If the spring rate is too soft for the variance in loading, it will lead to a harsh ride when loaded because of low uptravel and constant bottoming out. I ran into this issue with the Currie springs, particularly when loaded, that I only had as little as two inches of uptravel. (That problem was solved by airbags.)
Spring rate will also affect the total energy and the momentum absorbed before bottoming out. A stiffer spring can absorb significantly more momentum and energy as it is compressed to the limits of travel, at the cost of significantly increased restorative force on its ends.
Consider the two extremes for springs: constant rate and solid. A constant rate spring (think of a tape measure) would theoretically provide the perfect ride, but would accommodate zero changing load and would not restore height after a bump. A solid spring (think of a piece of pipe welded in) would eliminate the issues of sag due to load and bottoming out at the cost of extremely high restorative forces.
As for how that relates to the original poster, I agree with you that the springs are unlikely the issue here. Most springs sold for the TJ tend to be within a reasonable range of rates, with Currie being among the lightest and Rough Country among the heaviest if I recall correctly.
If the OP actually is bottoming out constantly, that is most likely the fault of blown or inadequate shocks. If it's more due to loading or even soft spring rates, stiffer springs or stiffening systems like airbags could be the solution. Ultra-stiff springs will likely give a noticeably harsher ride.
