David has the best answer. Which is to wire in some relays and switches to override the ECM method of controlling the fan, yet allow the ECM to override the relays for certain circumstances.
If you cut the #14 wire, the fan will start running as soon as battery power is available. As factoryblue pointed out, this puts additional stress on the electrical system during starting. A questionable starter or battery with the fan running can cause a slow crank.
Thrty5thSC is one of a few people that have cut the #14 wire, and never had a problem with the high speed fan coming on or if it did, it wasn't on sufficiently long to cause a problem. The concern with the high speed and low speed fan on at the same time is really only for people in areas or circumstances where it is really, really not outside.
So, if you want to do it right. Follow the diagram David provided. If you want to do it but don't have the skills or want to mess with it a lot, people have had success just cutting the wire, splicing in a switch, and then leave it on most of the time, and off when necessary. (Just don't forget to turn it back on)