That question has been asked and answered on this forum. If you use the forum's search feature here are just a couple of the threads you'll find...

http://ths.gardenweb.com/forums/load/plumbing/msg1014072014548.html

http://ths.gardenweb.com/forums/load/plumbing/msg0113114623895.html

sherriz - Why is it that you want to soften your water? While I don't recommend it, I can understand why some people might want to soften only the HOT water, but never only the COLD water (whomever recommended that to you has no idea what they are talking about). If you do not soften at least the water going to your hot water heater, you will have hardness scale build up in your hot water heater, causing it to fail prematurely. Your dishes will also come out of the dishwasher with scale and spots. Softening only the cold water is worse than no softening at all - you get all of the expense and salt-hauling of a softener, with NONE of the benefits of softened water.

If you soften only the hot water, you will still see hardness buildup in your showers and sinks and need to use more detergent in your washing machine to compensate for the hardness.

Best practice is to soften all of your water. The only reasonable exception is to tee off a line upstream from the softener for drinking water only.

Why would you only want half-softened water in the shower? That is crazy talk. Soften all of the water except outdoor supplies and add one or more tap for drinking water.