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 $124.66   3.847%   $129.4557   $129.46   $129.45   $129.50  
  Purchase #2 $83.64   4.825%   $87.6756   $87.68   $87.70   $87.70  
  Purchase #3 $9.27   7.746%   $9.9881   $9.99   $10.00   $10.00  
  Purchase #4 $8.52   10.500%   $9.4146   $9.41   $9.40   $9.40  
  Purchase #5 $107.80   13.237%   $122.0695   $122.07   $122.05   $122.10  
  Purchase #6 $132.43   12.568%   $149.0738   $149.07   $149.05   $149.10  
  Purchase #7 $18.64   8.126%   $20.1547   $20.15   $20.15   $20.20  
  7 purchase totals = $484.96       $527.8320   $527.83   $527.80   $528.00