[vettestr]

A dual contact (battery kill opens + and - side) switch is the most straight forward approach but a second method is available. The reason each side is opened on a dual contact switch is because the alternator can produce a feedback or enough amperage when running to keep the engine running (with a single side contact switch) or without battery supply voltage. A clue to the reason this happens is the footnote in the installation instructions for MSD ignitions (they supply a diode with their 6AL Ign.. control box) A simple diode installed in the battery supply wire run stops this RUN ON condition. This same diode when installed allows the use of a single sided kill switch. Without this diode a 2 or dual contact kill switch is needed to ensure the alt. does not allow a RUN ON condition. You can read the installation instructions on line from MSD web site to check out what I am saying but a diode is only a couple of bucks to purchase and much cheaper than a 2 sided kill switch. The price of a 2 sided contact switch is pretty small when you think of the big picture and if you just go $$$ crazy you can buy a diode and 2 contact switch.
Just poking fun here but I do like the idea myself about keeping the ground side of the cars electronics grounded or using a single sided switch. We all seem to install newer electronics on our toys and some of them are susceptible to voltage spikes that a continuous ground (not opened by kill switch) absorbs spikes better in my opinion. That is just my 2 cents and welcome thoughts of others about value of keeping things grounded.
Jeff C