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 $54.82   8.339%   $59.3914   $59.39   $59.40   $59.40  
  Purchase #2 $235.73   3.242%   $243.3724   $243.37   $243.35   $243.40  
  Purchase #3 $165.39   7.713%   $178.1465   $178.15   $178.15   $178.10  
  Purchase #4 $70.55   7.734%   $76.0063   $76.01   $76.00   $76.00  
  Purchase #5 $148.72   10.321%   $164.0694   $164.07   $164.05   $164.10  
  Purchase #6 $53.63   7.306%   $57.5482   $57.55   $57.55   $57.50  
  Purchase #7 $29.41   4.218%   $30.6505   $30.65   $30.65   $30.70  
  7 purchase totals = $758.25       $809.1847   $809.19   $809.15   $809.20