Do speaker cables have to rest on floor?

You don't need to have your speaker cables in a straight line. Where did you read that? Just keep them more than 6" off the floor and all will be fine. I use a 3M tape on wall hook for one side and a wire metal piece (intended for something else) for the other side to suspend the cables. 

It's no more clutter than the cables themselves and makes for easy cleaning of the carpets around them. It also helps to have not lots more cable than you need and therefore, not much more to suspend. 

All the best,
Nonoise

Ignore the audio police.

Let those cables lay on the ground and enjoy listening to music.

Yup off the floor makes it a lot easier to clean under. Remember the magic happen on the outside of the wire not the inside. The fields expands and contract. Do they sound better laying on the ground with static discharge and dust bunnies or off the ground with a narrow insulated lifter and vibration control? Cable routing is an art, it's just as important as the cable construct it's self..

BUT if your a hook, drop and let er flop, kind of person, I'd stick with XLR and watch the routing just to STAY out of trouble, instead of getting out of trouble..

Regards

Best is to install speaker cables in galvanized conduit secured at either end. This way they run absolutely straight in a slope from the amp up to the speakers. Nice straight line, no unsightly cable risers. If worried about stress have a metal shop arc weld them at both ends.