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 $104.81   12.305%   $117.7069   $117.71   $117.70   $117.70  
  Purchase #2 $235.27   6.191%   $249.8356   $249.84   $249.85   $249.80  
  Purchase #3 $53.60   13.146%   $60.6463   $60.65   $60.65   $60.60  
  Purchase #4 $170.65   9.178%   $186.3123   $186.31   $186.30   $186.30  
  Purchase #5 $56.48   12.370%   $63.4666   $63.47   $63.45   $63.50  
  Purchase #6 $51.51   6.792%   $55.0086   $55.01   $55.00   $55.00  
  Purchase #7 $12.92   4.756%   $13.5345   $13.53   $13.55   $13.50  
  7 purchase totals = $685.24       $746.5108   $746.52   $746.50   $746.40