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 $127.51   12.505%   $143.4551   $143.46   $143.45   $143.50  
  Purchase #2 $134.52   3.169%   $138.7829   $138.78   $138.80   $138.80  
  Purchase #3 $96.75   8.391%   $104.8683   $104.87   $104.85   $104.90  
  Purchase #4 $10.73   12.718%   $12.0946   $12.09   $12.10   $12.10  
  Purchase #5 $99.76   3.155%   $102.9074   $102.91   $102.90   $102.90  
  Purchase #6 $125.42   7.181%   $134.4264   $134.43   $134.45   $134.40  
  Purchase #7 $47.85   8.827%   $52.0737   $52.07   $52.05   $52.10  
  7 purchase totals = $642.54       $688.6084   $688.61   $688.60   $688.70