Propane level gauge-do I need one in each tank??

I had to purchase new tanks a couple of years ago, neither one have gauges on them (sorry, do not know where to buy an accurate gauge). What works best for us is operate off one and when auto switch valve goes to other tank I get the empty one filled then when it switches back I fill that one. Always have a full tank in reserve that way. Just the way I do it without gauges.