Is it necessary to change axis sensitivity and dead zone effect?

No. For many games, you may find that the default axis sensitivity ,  dead zone and range work for you. If you are not getting the response you expect when you use the axis to perform a command, you may want to try a different setting.

Is it necessary to change axis range?

No. This control should only be used in conjunction with mapping a gamepad axis to a mouse axis. In this scenario it can be used to generate a smaller amount of mouse movement for a given gamepad axis movement .i.e. it makes the mouse response less sensitive.

When I change these settings does it affect the commands assigned to both axis directions?

Yes. Changing either of these settings affects both directions of the axis command.