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 $237.53   8.182%   $256.9647   $256.96   $256.95   $257.00  
  Purchase #2 $85.93   3.396%   $88.8482   $88.85   $88.85   $88.80  
  Purchase #3 $220.60   8.749%   $239.9003   $239.90   $239.90   $239.90  
  Purchase #4 $97.42   7.402%   $104.631   $104.63   $104.65   $104.60  
  Purchase #5 $58.59   10.259%   $64.6007   $64.60   $64.60   $64.60  
  Purchase #6 $43.50   13.829%   $49.5156   $49.52   $49.50   $49.50  
  Purchase #7 $54.82   7.650%   $59.0137   $59.01   $59.00   $59.00  
  7 purchase totals = $798.39       $863.4742   $863.47   $863.45   $863.40