I don't know if it's safe or not, it depends on how much monitoring you are willing to do, switching it on/off.
You can get temp switches from just about any modern vehicle that will thread into coolant taps in the heads or wherever, it simply makes sense to use one. From my experience, without a heavy load on the engine, the fan only needs to run a minute or so to knock 15*+ off the engine temp.
Not to mention the fact that flow through cooling of the radiator is more effective than fans over a certain speed (assuming good condition cooling system) and your engine is a better judge of that speed than you are. A temp switch for a fan (GM anyways) usually swings about 15* from the on/off temp, that can happen OFTEN depending on driving conditions.
It's really safer to allow a temp switch to handle fan operation, and you monitor the gauges. You could still wire it up with a manual override of course, but that's kind of pointless...if you need the fan off, either turn the engine off, or pull the fuse, if you need it on all the time for whatever reason, ground the temp switch wire. But I can't think of any reason you'd want the fan on all the time (since the switch would do it if the engine was hot enough) and can't think of a common reason to want the fan forced off if the truck is on.