A thermo switch isnt "necessary" per sey ... but it will help your from having to remember to flip a switch to turn on the fan(s). You can wire it to be on when the ignition is on, but then it would run all the time even when it doesnt need to.
If i were you i would wire the fan(s) to come one with a thermo switch, or wire them to use a manual switch. Either way though, make sure that the fan(s) fan only get power when the ignition is on.
When i did my install i used a manual switch. The only problem is that when my GF drives my Jeep she often forgets to keep an eye on the gauges and turn the switch to low or high speed respectfully.