Since Ultra Low Sulfur Diesel lacks the lubicity of old time diesel it was suggested by my dealer (and several others) to ALWAYS use an additive like Howes or Power Service because they replace the lubricity required to prevent fuel system problems long term. Like you said, cheap insurance. I treat all of my diesel when I get it home (I buy in 5 gallon cans). As the bottles say, adding too much is okay too so if you're in doubt as to whether the fuel was treated, then add treatment.
As far as saving on the road taxes, in my state (NH) as long as you use more than 100 gallons of fuel per year in off-highway applications then you can submit a form with your receipts to get a reimbursement for the highway taxes. Off-highway applications includes the use of gasoline in landscaping equipment as well.
