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 $11.42   6.620%   $12.176   $12.18   $12.20   $12.20  
  Purchase #2 $25.60   9.301%   $27.9811   $27.98   $28.00   $28.00  
  Purchase #3 $132.56   6.470%   $141.1366   $141.14   $141.15   $141.10  
  Purchase #4 $192.95   10.199%   $212.629   $212.63   $212.65   $212.60  
  Purchase #5 $18.51   10.718%   $20.4939   $20.49   $20.50   $20.50  
  Purchase #6 $197.32   10.691%   $218.4155   $218.42   $218.40   $218.40  
  Purchase #7 $19.95   9.336%   $21.8125   $21.81   $21.80   $21.80  
  7 purchase totals = $598.31       $654.6446   $654.65   $654.70   $654.60