Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $30.25   3.683%   $31.3641   $31.36   $31.35   $31.40  
  Purchase #2 $16.92   6.391%   $18.0014   $18.00   $18.00   $18.00  
  Purchase #3 $61.99   13.271%   $70.2167   $70.22   $70.20   $70.20  
  Purchase #4 $200.25   9.614%   $219.502   $219.50   $219.50   $219.50  
  Purchase #5 $204.63   8.440%   $221.9008   $221.90   $221.90   $221.90  
  Purchase #6 $213.78   7.667%   $230.1705   $230.17   $230.15   $230.20  
  Purchase #7 $109.46   11.858%   $122.4398   $122.44   $122.45   $122.40  
  7 purchase totals = $837.28       $913.5953   $913.59   $913.55   $913.60