Gary has one opinion (as a softener salesman) and I have another (as a
water production utility laboratory employee). We've bumped heads in
the past and have agreed to disagree.

Gary has all sorts of numbers ready to show you how much money a
softener will save you over time. I don't quite trust those numbers and
don't see the need to add yet another device that will have to be
maintained and replaced (about as often as a water heater on hard
water) further down the road. Much cheaper in my opinion to just
replace the water heater when needed (every 10-15 years on hard
water... maybe, some people go as long as 25 years without problems).

If you do choose to soften, then I agree that only the hot side should
be treated. Again, Gary will have all sorts of number ready to show you
how much money you'll save by softening the whole house. Of course his
industry is also interested in selling you salt. By softening just the
hot side you'll save money on things that don't need softened (Gary
would disagree saying everything needs softened) and only dump half the
used-up salt down the drain as you would softening the whole house.

Another thing to consider. Removing the calcium from the water by
softening does remove scale. Gary would have you believe scale is BAD,
however it does provide a physical barrier that actually protects your
metallic pipes from the natural corrosive nature of water (think Grand
Canyon). This corrosive nature can be further enhanced by adding a
recirculating hot water pump to your hot water system. I've seen these
softened recirculating hot water system develop pin-hole leaks in as
little as five years. Talk about major money to fix!

Here's a little snippet from years past.

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As others have said, the salt loading (and I mean "salt", not calcium
chloride, as it take many times the amount of brine to "flush" the
calcium off the resin beads) probably won't hurt the septic tank itself
(although everything I've read says to limit the amount of water going
into the septic system and softeners dump a lot of water). However, you
may need to worry about collateral damage to trees and or shrubs that
may be in the drainage area. My Dad lost a 30 (40?) year old shag-bark
hickory a year after installing his softener and a blue spruce the
following year. Since both hickory and spruce don't mind wet feet (his
house is in a creek bottom) don't tell me it was the extra water that
killed it.
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One way to find out if what you're getting is scale or true sand is to
place your 'sand' in some vinegar. If it bubbles and disapears its
scale. If nothing happens it was real sand and not a produce of hard
water.