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 $235.49   11.870%   $263.4427   $263.44   $263.45   $263.40  
  Purchase #2 $25.71   4.503%   $26.8677   $26.87   $26.85   $26.90  
  Purchase #3 $124.61   3.797%   $129.3414   $129.34   $129.35   $129.30  
  Purchase #4 $56.52   5.379%   $59.5602   $59.56   $59.55   $59.60  
  Purchase #5 $63.76   5.262%   $67.1151   $67.12   $67.10   $67.10  
  Purchase #6 $8.44   13.451%   $9.5753   $9.58   $9.60   $9.60  
  Purchase #7 $50.29   10.859%   $55.751   $55.75   $55.75   $55.80  
  7 purchase totals = $564.82       $611.6534   $611.66   $611.65   $611.70