What to do with excess co-ax cable
Best would be to shorten the cables.  A shorter cable means less loss of signal in the cable, and a neater installation.  Doing so requires proper tools, new connectors, and some practice in attaching the connectors to the cables. That's too much to expect pilots working on their own planes, or even mechanics who do the work for many pilots.  There are simply too many differing connectors, cables, and tools.  Doing it wrong is worse than just leaving the cable a bit too long.
Leaving the cables at the stock length is the safest way.  The cables were assembled at the factory by technicians (or machines) that can do the job properly and the cables can be tested for proper function.


Don't worry about induction losses in the cables.  They are shielded.  There shouldn't be any measurable inductance just from coiling them up.  Whether you just coil them or make them into a figure eight shouldn't make any difference, either.

What you do need to be careful of is coiling them too tightly.  The cables are made of several layers of materials.  The proper function of the cable depends on the relationship of each layer to the other.

Here's a typical coaxial cable (from wikipedia.)



In the cables you are dealing with, the layers are thinner and more easily damaged.  A kink in the cable can cause the center conductor to squeeze through the inner insulator and either directly short it or allow the signal to more easily cross through to the outer conductor.  Either is bad.

Make your coils large, don't kink the cables, and you should be fine.