How important are BJT characteristic curves when designing amplifiers?
That's the reality of semiconductor manufacturing. Some parameters are really difficult to control tightly. Therefore, you design your circuits so that the overall circuit characteristics do not strongly depend on the actual characteristics of any particular device. Then, when you are done, you go back and verify that the circuit operation is acceptable over the full range of possible characteristics for each device.

It sounds complicated, but you'll soon learn how to subdivide the problem to keep things under control. You apply the same concept to multi-stage circuits &mdash; you design each stage so that its performance does not depend strongly on what is connected to its inputs and outputs.

For example, in an amplifier, you use negative feedback to set the gain of the circuit so that it is not a strong function of the actual current transfer ratio of the transistor. This could be as simple as putting a resistor in series with the emitter.