If you are planning on racing at any sactioned race events, or just want an extra safety precaution, then a tether switch is the way to go.  You can pick one up at your local Arctic Cat dealer for around $25.00.  They are very easy to install, and give your sled added protection incase you should fall off.  Below are the instructions of how and where we mounted ours:
F7 Tether Switch Installation
This page was last updated on: August 4, 2010
Approx. Time:  30 Mins

*Tools and Hardware Needed:
1 - AC Tether Switch
1 - 1/2" Drill Bit
1 - Pliers
1 - Measuring Tape or Ruler

Step 1:
Measure 2 1/4" from key boss (see Pic 1).

Step 2:
Push tether body through hole, and screw cap back on.  Tighten with pliers.

Step 3:
Put rubber on cap.  Make sure rubber fits snug.

Step 4:
Disconnect black wire from back of key switch.

Step 5:
Connect black wire to brown pig-tail connector (in AC tether switch kit). 

Step 6:
Connect one brown wire to black and white wire from tether switch and second brown wire to key switch spade.

Step 7:
Disconnect purple wire from key switch.

Step 8:
Connect purple wire to purple pig-tail connector (in AC tether switch kit).

Step 9:
Connect one purple wire to black wire from tether switch and second purple wire to key switch spade.

Step 10:
Test.  Start engine and pull tether cord off.  If engine does not stop, check wire connections.

Pic 1 - Key Boss Measurement