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 $28.52   6.484%   $30.3692   $30.37   $30.35   $30.40  
  Purchase #2 $14.97   7.376%   $16.0742   $16.07   $16.05   $16.10  
  Purchase #3 $117.69   7.363%   $126.3555   $126.36   $126.35   $126.40  
  Purchase #4 $45.86   8.568%   $49.7893   $49.79   $49.80   $49.80  
  Purchase #5 $127.49   9.794%   $139.9764   $139.98   $140.00   $140.00  
  Purchase #6 $77.79   11.208%   $86.5087   $86.51   $86.50   $86.50  
  Purchase #7 $125.66   5.650%   $132.7598   $132.76   $132.75   $132.80  
  7 purchase totals = $537.98       $581.8331   $581.84   $581.80   $582.00